We are offering 3,816,793 shares of our common stock. Our common stock is listed for trading on The Nasdaq Global Market under the symbol "OMER." On December 4, 2019, the last reported sale price of our common stock on The Nasdaq Global Market was $15.24 per share.

We have granted Cantor Fitzgerald & Co. an option for a period of 30 days to purchase up to 572,518 additional shares of our common stock at the price set forth above. If Cantor Fitzgerald & Co. exercises the opinion in full, the total proceeds to us, before expenses, will be approximately $54.5 million.

Investing in our common stock involves significant risks. Before buying shares of our common stock, you should carefully consider the risks described under the caption "Risk Factors" beginning on page S-7 of this prospectus supplement and in the documents incorporated by reference into this prospectus supplement.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the accuracy or adequacy of this prospectus supplement or the accompanying prospectus. Any representation to the contrary is a criminal offense.

The underwriter expects to deliver the shares of common stock against payment on or about December 9, 2019.

This document is in two parts. The first part is this prospectus supplement, which describes the specific terms of this offering of common stock and updates the information contained in the accompanying prospectus and the documents incorporated by reference herein and therein. The second part is the accompanying prospectus, which provides more general information, some of which does not apply to this offering. If information included or incorporated by reference in this prospectus supplement is inconsistent with the accompanying prospectus or the information incorporated by reference therein, then this prospectus supplement or the information incorporated by reference in this prospectus supplement will apply and will supersede the information in the accompanying prospectus and the documents incorporated by reference therein. For a more detailed understanding of our common stock, you should read both this prospectus supplement and the accompanying prospectus, together with additional information described under the heading "Where You Can Find More Information."

This prospectus supplement is part of a shelf registration statement on Form S-3 that we filed with the U.S. Securities and Exchange Commission (the "SEC") on December 4, 2019. Under the shelf registration process, we may from time to time offer and sell any combination of the securities described in the accompanying prospectus.

Neither we nor the underwriter have authorized anyone to provide you with information that is different or in addition to that contained or incorporated by reference in this prospectus supplement, the accompanying prospectus or any free writing prospectus prepared by us or on our behalf. Neither we nor the underwriter take any responsibility for, and can provide no assurance as to the reliability of, any information that others may give. Neither we nor the underwriter are making an offer to sell or soliciting an offer to buy our securities under any circumstance in any jurisdiction where the offer or solicitation is not permitted. You should assume that the information contained in this prospectus supplement, the accompanying prospectus and any free writing prospectus prepared by us or on our behalf is accurate only as of the date of the respective document in which the information appears, and that any information in documents that we have incorporated by reference is accurate only as of the date of the document incorporated by reference, regardless of the time of delivery of this prospectus supplement or any sale of a security. Our business, financial condition, results of operations and prospects may have changed since those dates.

This prospectus supplement, the accompanying prospectus, and the information incorporated herein and therein by reference includes trademarks, service marks and trade names owned by us or other companies. All trademarks, service marks and trade names included or incorporated by reference into this prospectus supplement or the accompanying prospectus are the property of their respective owners.

Unless the context indicates otherwise, in this prospectus supplement and the accompanying prospectus the terms "Omeros," "we," "us" and "our" refer to Omeros Corporation, a Washington corporation, and its subsidiaries on a consolidated basis.

This prospectus supplement, the accompanying prospectus, and the information incorporated by reference in this prospectus supplement and the accompanying prospectus contain "forward-looking statements" within the meaning of Section 27A of the Securities Act of 1933 (the "Securities Act") and Section 21E of the Securities Exchange Act of 1934 (the "Exchange Act"), which are subject to the "safe harbor" created by those sections for such statements. Forward-looking statements are based on our management's beliefs and assumptions and on information currently available to them. All statements other than statements of historical fact are forward- looking statements. Terms such as "anticipate," "believe," "could," "estimate," "expect," "goal," "intend," "likely," "may," "plan," "potential," "predict," "project," "should," "target," "will," "would" and similar expressions and variations thereof are intended to identify forward-looking statements, but these terms are not the exclusive means of identifying such statements. Those statements appear in this prospectus supplement, the accompanying prospectus and the documents incorporated herein and therein by reference, particularly in the sections entitled "Prospectus Summary" and "Risk Factors," and include statements regarding the intent, belief or current expectations of us and our management that are subject to known and unknown risks, uncertainties and assumptions. Examples of these statements include, but are not limited to, statements regarding:

Our actual results could differ materially from those anticipated in these forward-looking statements for many reasons, including the risks, uncertainties and other factors described in this prospectus supplement, the accompanying prospectus and the documents incorporated herein and therein by reference, particularly in the sections entitled "Risk Factors" and "Management's Discussion and Analysis of Financial Condition and Results of Operations." Given these risks, uncertainties and other factors, actual results or anticipated developments may not be realized or, even if substantially realized, may not have the expected consequences to or effects on our company, business or operations. Accordingly, you should not place undue reliance on these forward-looking statements, which represent our estimates and assumptions only as of the date they are made. You should read this prospectus supplement, the accompanying prospectus and the information incorporated by reference herein and therein with the understanding that our actual results in subsequent periods may materially differ from current expectations. Except as required by law, we assume no obligation to update or revise any forward-looking statements contained herein, whether as a result of any new information, future events or otherwise.

PROSPECTUS SUPPLEMENT SUMMARY

This summary highlights some of the information contained elsewhere in this prospectus supplement, the accompanying prospectus, and the documents incorporated by reference herein and therein. It is not complete and may not contain all of the information you should consider before making an investment decision. Before you decide to invest in our securities, you should read the entire prospectus supplement and the accompanying prospectus carefully, including the risk factors and the financial statements and related notes included or incorporated by reference herein and therein.

Company Overview

We are a commercial-stage biopharmaceutical company committed to discovering, developing and commercializing small-molecule and protein therapeutics for large-market as well as orphan indications targeting inflammation, complement-mediated diseases, disorders of the central nervous system, and immune-related diseases, including cancers.

Our drug product OMIDRIA® is marketed in the United States for use during cataract surgery or intraocular lens replacement to maintain pupil size by preventing intraoperative miosis (pupil constriction) and to reduce postoperative pain. In our pipeline we have multiple Phase 3 and Phase 2 clinical-stage development programs focused on complement-mediated disorders, including HSCT-TMA, IgA nephropathy and aHUS, and substance abuse. In addition, we have a diverse group of preclinical programs, including GPR174, a novel target in immuno-oncology that modulates a new cancer immunity axis recently discovered by us. Small-molecule inhibitors of GPR174 are part of our proprietary G protein-coupled receptor ("GPCR") platform through which we control 54 new GPCR drug targets and their corresponding compounds. We also exclusively possess a novel antibody-generating platform. For OMIDRIA and each of our product candidates and our programs, we have retained control of all commercial rights.

Commercial Product

OMIDRIA is a proprietary drug product containing two active pharmaceutical ingredients: ketorolac, an anti-inflammatory agent, and phenylephrine, a mydriatic, (pupil dilating), agent. Cataract and other lens replacement surgery involves replacement of the original lens of the eye with an artificial intraocular lens. These procedures are typically performed to replace a lens opacified by a cataract and/or to correct a refractive error. OMIDRIA is added to standard irrigation solution used during cataract and lens replacement surgery and is delivered intracamerally, or within the anterior chamber of the eye, to the site of the surgical trauma throughout the procedure. Preventing pupil constriction is essential for these procedures and, if miosis occurs, the risk of damaging structures within the eye and other complications increases, as does the operating time required to perform the procedure.

We launched OMIDRIA in the U.S. in the second quarter of 2015 and sell OMIDRIA primarily through wholesalers which, in turn, sell to ASCs and hospitals. CMS, the federal agency responsible for administering the Medicare program, granted transitional pass-through reimbursement status for OMIDRIA in 2014, effective from January 1, 2015 through December 31, 2017. Pass-through status allows for separate payment (i.e., outside the packaged payment rate for the surgical procedure) under Medicare Part B. In March 2018, the Consolidated Appropriations Act of 2018 (the "Appropriations Act") was signed into law. The Appropriations Act included a provision by which Congress extended pass-through reimbursement status for a small number of drugs, including OMIDRIA, used during procedures performed on Medicare Part B fee-for-service patients for an additional two years, running from October 1, 2018 until September 30, 2020.

We continue to pursue permanent separate reimbursement for OMIDRIA beyond the currently scheduled expiration of pass-through reimbursement. CMS is required under the Substance Use—

Disorder Prevention that Promotes Opioid Recovery and Treatment (SUPPORT) for Patients and Communities Act to review payments under its CMS' outpatient prospective payment system ("OPPS") for opioids and evidence-based non-opioid alternatives for pain management with a goal to ensure that there are not financial incentives to use opioids instead of non-opioid alternatives. In its 2020 OPPS proposed rule, CMS noted that non-opioid drugs that are indicated for reduction of post-operative pain may warrant separate payment if there is evidence to show that such drugs help to deter or avoid prescription opioid use and addiction and that packaged payment presents a demonstrated barrier to access for such drugs. Although Omeros provided CMS with evidence that it believes shows that OMIDRIA meets these criteria, CMS declined in its 2020 OPPS final rule to grant separate payment to OMIDRIA beyond the expiration of its current pass-through status on September 30, 2020. CMS also noted in the 2020 final rule that it will continue to analyze evidence and monitor utilization of OMIDRIA. We continue to generate evidence and intend to continue pursuing administrative and legislative avenues to secure permanent separate payment or similar reimbursement for OMIDRIA beyond September 30, 2020; however, we cannot provide assurance that these efforts will be successful.

We also continue to pursue expansion of reimbursement for OMIDRIA by Medicare Advantage and other third-party payers. CMS recently assigned a permanent product-specific Healthcare Common Procedure Coding System ("HCPCS"), J-code for OMIDRIA, which became effective October 1, 2019 and replaced the drug's former temporary HCPCS C-code. J-codes are reimbursement codes used by commercial insurance plans, Medicare, Medicare Advantage, and other government payers for drugs like OMIDRIA that are administered by a physician. Benefits of the new J-code include having one consistent billing code that can be used across all government and commercial payer plans as well as expanding coverage in state Medicaid and commercial plans that recognize J-codes, but not C-codes, for reimbursement and enabling access to the increasing number of cataract procedures performed in the physician office setting.

Our Product Candidates and Development Programs

Our clinical product candidates consist of the following:

Product Candidate	Targeted Disease	Development Status	Next Expected Milestone	Worldwide Rights

*Clinical Programs*
Narsoplimab (OMS721/MASP-2)—Lectin Pathway Disorders	HSCT-TMA	Phase 3	Complete Rolling Submission of BLA and Submit MAA	Omeros (In-licensed)
Narsoplimab (OMS721/MASP-2)—Lectin Pathway Disorders	IgA Nephropathy	Phase 3	Complete Phase 3 Patient Enrollment or Perform 36-Week Assessment of Proteinuria	Omeros (In-licensed)
Narsoplimab (OMS721/MASP-2)—Lectin Pathway Disorders	aHUS	Phase 3	Complete Phase 3 Patient Enrollment	Omeros (In-licensed)
Narsoplimab (OMS721/MASP-2)—Lectin Pathway Disorders	Lupus Nephritis and Other Renal Diseases	Phase 2	Review Data; Determine Whether to Initiate Phase 3 Program	Omeros (In-licensed)
PDE7 (OMS527)	Addictions and Compulsive Disorders; Movement Disorders	Phase 1	Initiate Phase 2a Study	Omeros (Compounds In-licensed)
PPARg (OMS405)—Addiction	Opioid and Nicotine Addiction	Phase 2	Further Refine Development Path	Omeros

Our pipeline of development programs consist of the following:

Pipeline Program	Targeted Disease	Development Status	Next Expected Milestone	Worldwide Rights

*Development Programs and Platforms*
MASP-3 (OMS906)—Alternative Pathway Disorders	Paroxysmal Nocturnal Hemoglobinuria and Other Alternative Pathway Disorders	Preclinical	Complete IND-Enabling Toxicology Studies	Omeros
MASP-2—Small Molecule Inhibitors	aHUS, IgA Nephropathy, HSCT,-TMA and Age-Related Macular Degeneration	Preclinical	Optimize Compounds	Omeros (In-Licensed)
MASP-2—Second Generation Antibody	Long-acting Second Generation Antibody Targeting Lectin Pathway Disorders	Preclinical	Optimize Compounds	Omeros
MASP-3—Small Molecule Inhibitors	PNH and Other Alternative Pathway Disorders	Preclinical	Continue Medicinal Chemistry and Advance Co-Crystallization Efforts	Omeros
GPCR Platform, including GPR 174, GPR151, GPR161 and other Class A Orphan GPCRs	Immunologic, Immuno-oncologic, Metabolic, CNS, Cardiovascular, Musculoskeletal & Other Disorders	Preclinical	Continue Drug Discovery and Selected Medicinal Chemistry for Class A Orphan and Class B Orphan and Non-Orphan GPCRs	Omeros
Antibody Platform	Metabolic, CV, Oncologic, Musculoskeletal & Other Disorders	Preclinical	Continue Developing Antibodies Targeting Lectin and Alternative Pathway of Complement System and Expanding Antibody Library	Omeros (In-licensed)

Recent Events

Clinical Findings in Pivotal Trial of HSCT-TMA Patients Treated with Narsoplimab

On December 4, 2019, we announced positive data in our pivotal clinical trial of our investigational complement inhibitor narsoplimab in the treatment of HSCT-TMA, a frequently lethal complication of HSCT. These preliminary data were recently provided to FDA as part of our ongoing interactions with the agency regarding our rolling BLA submission. The single-arm, open-label trial included safety and efficacy endpoints that were established following discussions with FDA. These endpoints were assessed for (i) all patients who received at least one dose of narsoplimab and (ii) patients who received at least four weeks of narsoplimab dosing.

The primary efficacy endpoint in the trial was the proportion of patients who achieved designated "responder" status based on improvement in HSCT-TMA laboratory markers and clinical status. The primary laboratory markers that were evaluated were platelet count and lactate dehydrogenase ("LDH") levels, while improvement in clinical status was evaluated based on organ function and transfusions. Patients who did not fully meet these criteria were considered "non-responders." Among patients who received at least one dose of narsoplimab, 56% achieved responder status, while 68% of patients who received the protocol-specified narsoplimab treatment of at least four weeks of dosing achieved responder status.

Secondary endpoints in the trial were survival rates and change from baseline in HSCT-TMA laboratory markers. Among the responder population, 93% of patients survived for at least 100 days following HSCT-TMA diagnosis, while 81% of patients who received treatment for at least four weeks and 65% of the total treated population achieved this benchmark. Results also included statistically significant improvements in platelet count, LDH and haptoglobin (p < 0.01 for all treated patients, p £ 0.002 for patients who received treatment for at least four weeks and p < 0.001 for responders). In addition, hemoglobin increased in all groups and reached statistical significance (p = 0.041) in responders. Creatinine also improved in all patient groups, but did not reach statistical significance, we believe due to the use of nephrotoxic agents in trial patients.

The treated population had multiple high-risk features that portend a poor outcome, including the persistence of HSCT-TMA despite modification of immunosuppression (which was a criterion for entry into the trial), graft-versus-host disease, significant infections, non-infectious pulmonary complications and neurological findings. Patients in the trial had a high expected mortality rate, with 93% of them having multiple risk factors. No signal of any serious safety risk was observed with narsoplimab in the trial. The most common adverse events observed in the trial were nausea, vomiting, diarrhea, hypokalemia, neutropenia and fever, which are all common in stem-cell transplant patients. 21% of patients died during the trial due to causes common in stem cell transplant, with no additional patients discontinuing for adverse events. The data from the patients who died were not excluded from any analyses.

Enrollment of patients in this trial has completed. The details of the endpoints, including the response criteria agreed with FDA, and the number of patients in the trial remain confidential for competitive business reasons.

Preclinical Findings on GPR 174

On November 19, 2019, we announced new findings on GPR174, our preclinical program targeting immuno-oncology. Our studies in mouse models of melanoma and colon carcinoma found that GPR174-deficiency resulted in significantly reduced tumor growth and improved survival of the animals (p=0.006 in melanoma; p=0.03 in colon cancer) versus normal mice. We have presented these findings at the American Association for Cancer Research Conference on Tumor Immunology and Immunotherapy.

Contract Arbitration

On December 3, 2019, we received a demand for arbitration from a former manufacturer in connection with a contract dispute relating to failed or cancelled batches of OMS721 under a 2011 development and manufacturing agreement. The former manufacturer claims it is owed approximately $6.76 million for manufacturing milestones, raw materials and cancellation fees under the agreement, plus interest and legal fees. We dispute that these amounts are owed and intend to seek return of approximately $2.95 million in prior payments to the manufacturer. This dispute does not affect clinical or commercial supplies of narsoplimab or any of our products or other product candidates, which are currently produced by other contract manufacturers.

Company Information

We were incorporated in the State of Washington in 1994. Our principal executive offices are located at 201 Elliott Avenue West, Seattle, Washington 98119, and our telephone number is (206) 676-5000. Our website address is www.omeros.com. The information on, or that can be accessed through, our website is not part of this prospectus supplement or the accompanying prospectus.

THE OFFERING


Common stock offered by us		3,816,793 shares
Underwriter's option to purchase additional shares		The underwriter has an option to purchase up to 572,518 additional shares of our common stock, which they may exercise, in whole or in part, for a period of 30 days from the date of this prospectus supplement.
Common stock to be outstanding immediately after this offering		53,342,267 shares, or 53,914,785 shares if the underwriter's option to purchase additional shares is exercised in full.
Use of proceeds		We intend to use the net proceeds of the offering for general corporate purposes, including funding clinical trials, pre-clinical studies, manufacturing, the build-out of commercial infrastructure and other costs associated with advancing our development programs and product candidates toward regulatory submissions and potential commercialization. We may also use the net proceeds for working capital, the repayment of debt obligations, acquisitions or investments in businesses, products or technologies that are complementary to our own, and other capital expenditures. See "Use of Proceeds" on page S-9 of this prospectus supplement.
Risk factors		Investing in our common stock involves significant risks. See "Risk Factors" beginning on page S-7 of this prospectus supplement.
Nasdaq Global Market listing		Our common stock is listed on The Nasdaq Global Market under the symbol "OMER."

Outstanding Shares

The number of shares of our common stock to be outstanding immediately after this offering, as stated above, is based on 49,525,474 shares outstanding as of September 30, 2019 and excludes as of that date:

•: 11,494,722 shares of common stock issuable upon the exercise of outstanding stock options with a weighted-average exercise price of $11.67 per share;
•: 243,115 shares of common stock issuable upon the exercise of outstanding warrants with a weighted-average exercise price of $20.68 per share; and
•: 5,396,836 shares of common stock available for future grants under our 2017 Omnibus Incentive Compensation Plan.

In addition, the number of shares to be outstanding immediately after this offering does not include shares of common stock issuable upon conversion of our 6.25% Convertible Senior Notes due 2023 (the "Notes"), to the extent we elect to settle conversions of the Notes in shares of common stock. The conversion price of the Notes is $19.22 per share, but due to the capped call transaction we entered into in connection with the issuance of the Notes, we will only be required to issue shares of common stock or make cash payments to settle conversions to the extent the market price of our common stock at the time of conversion is greater than $28.8360.

Except as otherwise indicated, all information in the prospectus supplement, including the number of shares of common stock outstanding immediately after this offering, assumes no exercise by the underwriter of its option to purchase additional shares of common stock.

An investment in our common stock involves a high degree of risk. Before deciding whether to invest in our common stock, you should consider carefully the risks described below and discussed under the section captioned "Risk Factors" in Part I, Item 1A of our Annual Report on Form 10-K for the year ended December 31, 2018, which is incorporated by reference in this prospectus supplement and the accompanying prospectus in its entirety, together with other information in this prospectus supplement, the accompanying prospectus, and the information and documents incorporated by reference that we have authorized for use in connection with this offering. If any of these risks actually occur, our business, financial condition, results of operations or cash flows could be seriously harmed. This could cause the trading price of our common stock to decline, resulting in a loss of all or part of your investment.

Our stock price has been and may continue to be volatile, and the value of an investment in our common stock may decline.

During the 12-month period ended September 30, 2019, our stock traded as high as $24.87 per share and as low as $10.30 per share. The trading price of our common stock is likely to continue to be highly volatile and could be subject to wide fluctuations in response to numerous factors, many of which are beyond our control. In addition, the stock market has experienced extreme price and volume fluctuations that have often been unrelated or disproportionate to the operating performance of publicly traded companies. Broad market and industry factors may seriously affect the market price of companies' stock, including ours, regardless of actual operating performance. These fluctuations may be even more pronounced in the trading market for our stock. In addition, in the past, following periods of volatility in the overall market and the market price of a particular company's securities, securities class action litigation has often been instituted against these companies. This litigation, if instituted against us, could result in substantial costs and a diversion of our management's attention and resources.

You will experience immediate and substantial dilution in the net tangible book value per share of the common stock you purchase.

Since the price per share of our common stock being offered is substantially higher than the net tangible book value per share of our common stock, your interest will be diluted to the extent of the difference between the price per share you pay and the net tangible book value per share of our common stock. After giving effect to the sale of 3,816,793 shares of our common stock in this offering at the public offering price of $13.10 per share and based on our net tangible book value as of September 30, 2019, if you purchase shares of common stock in this offering you will suffer substantial and immediate dilution of $14.84 per share in the net tangible book value of the common stock. The future exercise of outstanding options and warrants will result in further dilution of your investment. See the section entitled "Dilution" below for a more detailed discussion of the dilution you will incur if you purchase common stock in this offering.

Our management will have broad discretion as to the use of proceeds from this offering, and we may not use the proceeds effectively.

Our management will have broad discretion in the application of the net proceeds from this offering and could spend the proceeds in ways that do not improve our results of operations or enhance the value of our common stock. You will not have the opportunity, as part of your investment decision, to assess whether these proceeds are being used appropriately. Our failure to apply these funds effectively could have a material adverse effect on our business, delay the further development of OMIDRIA and our product candidates and cause the price of our common stock to decline.

If we issue additional shares of our common stock or other securities that may be convertible into, or exercisable or exchangeable for, our common stock, our existing shareholders would experience further dilution.

To the extent that we raise additional funds in the future by issuing equity securities, our shareholders would experience dilution, which may be significant and could cause the market price of our common stock to decline significantly. In addition, approximately 11.5 million shares of common stock as of September 30, 2019 subject to outstanding options and warrants may become eligible for sale in the public market to the extent permitted by the provisions of various vesting agreements. Further, as of September 30, 2019, we also had approximately 5.4 million shares of common stock reserved for future issuance under our employee benefit plans that are not subject to outstanding options. If the holders of these outstanding options or warrants elect to exercise some or all of them, or if the shares subject to our employee benefit plans are issued and become eligible for sale in the public market, our shareholders would experience dilution and the market price of our common stock could decline.

Anti-takeover provisions in our charter documents and under Washington law could make an acquisition of us, which may be beneficial to our shareholders, difficult and prevent attempts by our shareholders to replace or remove our current management.

Provisions in our articles of incorporation and bylaws and under Washington law may delay or prevent an acquisition of us or a change in our management. These provisions include a classified board of directors, a prohibition on shareholder actions by less than unanimous written consent, restrictions on the ability of shareholders to fill board vacancies and the ability of our board of directors to issue preferred stock without shareholder approval. In addition, because we are incorporated in Washington, we are governed by the provisions of Chapter 23B.19 of the Washington Business Corporation Act, which, among other things, restricts the ability of shareholders owning 10% or more of our outstanding voting stock from merging or combining with us. Although we believe these provisions collectively provide for an opportunity to receive higher bids by requiring potential acquirers to negotiate with our board of directors, they would apply even if an offer may be considered beneficial by some shareholders. In addition, these provisions may frustrate or prevent any attempts by our shareholders to replace or remove our current management by making it difficult for shareholders to replace members of our board of directors, which is responsible for appointing the members of our management.

We have never declared or paid dividends on our capital stock, and we do not anticipate paying dividends in the foreseeable future.

Our business requires significant funding. We currently plan to invest all available funds and future earnings, if any, in the development and growth of our business. Additionally, under the Silicon Valley Bank Loan and Security Agreement, we have agreed not to pay any dividends. Therefore, we currently do not anticipate paying any dividends on our common stock in the foreseeable future. As a result, a rise in the market price of our common stock, which is uncertain and unpredictable, will be the sole source of potential gain for shareholders in the foreseeable future, and an investment in our common stock for dividend income should not be relied upon.

We estimate that the net proceeds from the sale of 3,816,793 shares of common stock that we are offering will be approximately $47.1 million, or approximately $54.2 million if the underwriter exercises in full its option to purchase up to 572,518 additional shares of common stock, after deducting our estimated offering expenses.

We intend to use the net proceeds of the offering for general corporate purposes, including funding clinical trials, pre-clinical studies, manufacturing, build-out of commercial infrastructure and other costs associated with advancing our development programs and product candidates toward regulatory submissions and potential commercialization. We may also use the net proceeds for working capital, the repayment of debt obligations, acquisitions or investments in businesses, products or technologies that are complementary to our own, and other capital expenditures. Our management will have broad discretion in the application of the net proceeds received from this offering. Pending use of the net proceeds of this offering as described above, we intend to invest the net proceeds in short-term, investment-grade, interest-bearing instruments.

If you purchase our common stock in this offering, your interest will be diluted to the extent of the difference between the price per share you pay and the net tangible book value per share of our common stock immediately after this offering. Our net tangible book deficit as of September 30, 2019 was approximately $139.9 million, or $2.83 per share. Net tangible book deficit per share is determined by dividing our total tangible assets, less total liabilities, by the number of shares of our common stock outstanding as of September 30, 2019. Dilution with respect to net tangible book value per share represents the difference between the amount per share paid by purchasers of shares of common stock in this public offering and the net tangible book value per share of our common stock immediately after this public offering.

After giving effect to the sale of 3,816,793 shares of our common stock at an assumed public offering price of $13.10 per share, and after deducting the underwriting discount and estimated offering expenses payable by us, our adjusted net tangible book value as of September 30, 2019 would have been approximately $(92.8) million, or $(1.74) per share. This represents an immediate increase in the net tangible book value of $1.09 per share to existing shareholders and immediate dilution in net tangible book value of $14.84 per share to investors purchasing our common stock in this offering at the assumed public offering price.

The amounts in the table below assume no exercise by the underwriter of its option to purchase additional shares of common stock.

If the underwriter exercises in full its option to purchase up to 572,518 additional shares of our common stock, the as adjusted net tangible book value as of September 30, 2019 after giving effect to this offering would have been approximately $(1.59) per share, representing an increase in net tangible book value of $1.24 per share to existing shareholders and immediate dilution in net tangible book value of $14.69 per share to investors at the price for which we sold the shares to the underwriter.

Notwithstanding the assumptions reflected in this table, the shares sold in this offering, if any, will be sold from time to time at various prices. The dilution per share to new investors purchasing our common stock in this offering will depend on the number and price of shares of our common stock that are sold in this offering.

The number of shares of our common stock to be outstanding immediately after this offering, as reported above, is based on 49,525,474 shares outstanding as of September 30, 2019, and excludes as of that date:

In addition, the number of shares to be outstanding immediately after this offering does not include shares of common stock issuable upon conversion of the Notes, to the extent we elect to settle conversions of the Notes in shares of common stock. The conversion price of the Notes is $19.22 per share, but due to the capped call transaction we entered into in connection with the issuance of the Notes, we will only be required to issue shares of common stock or make cash payments to settle conversions to the extent the market price of our common stock at the time of conversion is greater than $28.8360.

To the extent that outstanding options or warrants as of September 30, 2019 have been or may be exercised or other shares issued, investors purchasing our common stock in this offering may experience further dilution. We also may choose to raise additional capital due to favorable market conditions or strategic considerations even if we believe that we have sufficient funds for our current or future operating plans. To the extent that we raise additional capital through the sale of equity or convertible debt securities, the issuance of these securities could result in further dilution to our shareholders.

We have never declared or paid any cash dividends on our capital stock, and under the Silicon Valley Bank Loan and Security Agreement, we have agreed not to pay any dividends. We expect to retain all available funds and future earnings, if any, to fund the development and growth of our business and we do not anticipate paying any cash dividends in the foreseeable future.

The following discussion describes material United States federal income and estate tax considerations relating to the acquisition, ownership, and disposition of shares by a non-U.S. Holder (as defined below) that acquires our shares in this offering and holds them as a capital asset. This discussion is based on the tax laws of the United States, including the Internal Revenue Code of 1986, as amended (the "Code"), Treasury regulations promulgated or proposed thereunder, and administrative and judicial interpretations thereof, all as in effect on the date hereof. These tax laws are subject to change, possibly with retroactive effect, and subject to differing interpretations that could affect the tax consequences described herein. This section does not address the treatment of a non-U.S. holder under the laws of any state, local or foreign taxing jurisdiction.

For purposes of this discussion, a "non-U.S. holder" is a beneficial owner of our shares that, for United States federal income tax purposes, is:

This discussion does not address all aspects of United States federal income taxation that may be applicable to holders in light of their particular circumstances or status (including, for example, banks and other financial institutions, insurance companies, brokers and dealers in securities or currencies, traders that have elected to mark securities to market, regulated investment companies, real estate investment trusts, partnerships or other pass-through entities, corporations that accumulate earnings to avoid United States federal income tax, tax-exempt organizations, pension plans, persons that hold our shares as part of a straddle, hedge or other integrated investment, persons subject to alternative minimum tax, certain United States expatriates, and foreign governments or agencies).

If a partnership (including any entity or arrangement treated as a partnership for United States federal income tax purposes) holds our shares, the tax treatment of a person treated as a partner in the partnership for United States federal income tax purposes generally will depend on the status of the partner and the activities of the partnership. Partnerships (and other entities or arrangements so treated for United States federal income tax purposes) and their partners should consult their own tax advisors.

This discussion addresses only non-U.S. holders and does not discuss any tax considerations other than United States federal income tax and certain United States federal estate tax considerations. Prospective investors are urged to consult their own tax advisors regarding the United States federal, state and local, and foreign tax consequences of the purchase, ownership, and disposition of our shares, including the effect of any applicable income or estate tax treaty.

As discussed above, we do not currently expect to pay dividends. If we do make a distribution of cash or property with respect to our shares, any such distribution generally will constitute a dividend for United States federal income tax purposes. Subject to the discussion below under "—FATCA Withholding" and "—Information Reporting and Backup Withholding," and except as described below, any such dividends paid to a non-U.S. holder generally will be subject to withholding tax at a 30% rate

or at a lower rate under an applicable income tax treaty between the United States and the non-U.S. holder's country of residence. In order to receive a reduced treaty withholding tax rate and to avoid backup withholding, as described below, a non-U.S. holder must furnish to us or our paying agent a properly executed Internal Revenue Service Form W-8BEN or Form W-8BEN-E (or other applicable form) prior to payment of the dividend, certifying under penalties of perjury that the non-U.S. holder is entitled to a reduction in withholding under an applicable income tax treaty. A non-U.S. holder that holds our shares through a financial institution or other agent will be required to provide appropriate documentation to the financial institution or other agent, which then will be required to provide certification to us or our paying agent either directly or through other intermediaries. A non-U.S. holder that is eligible for a reduced rate of withholding tax pursuant to an income tax treaty may obtain a refund of any excess amounts withheld by filing a refund claim with the Internal Revenue Service.

If a dividend is "effectively connected" with the conduct of a trade or business in the United States of the non-U.S. holder (and, if an applicable income tax treaty so provides, attributable to a permanent establishment or fixed base maintained by the non-U.S. holder in the United States), the non-U.S. holder is exempt from the withholding tax described above provided that the holder has furnished to us or our paying agent an Internal Revenue Service Form W-8ECI (or applicable successor form), certifying under penalties of perjury that the dividend is effectively connected with the non-U.S. holder's conduct of a trade or business in the United States (and, if an applicable income tax treaty so provides, attributable to a permanent establishment or fixed base maintained in the United States). "Effectively connected" dividends will generally be subject to United States federal income tax at the graduated rates that also apply to U.S. persons. A corporate non-U.S. holder may, under certain circumstances, be subject to an additional branch profits tax at a 30% rate (or at a lower rate under an applicable income tax treaty) with respect to its "effectively connected" dividends.

To the extent a distribution with respect to our shares exceeds our current or accumulated earnings and profits, as determined under United States federal income tax principles, the distribution will be treated, first, as a tax-free return of the non-U.S. holder's investment, up to the holder's adjusted tax basis in its shares, and, thereafter, as capital gain, which is subject to the tax treatment described below in "—Gain on Sale, Exchange or Other Taxable Disposition."

Subject to the discussion below under "—FATCA Withholding" and "—Information Reporting and Backup Withholding," a non-U.S. holder generally will not be subject to United States federal income tax or withholding tax on gain realized upon a sale, exchange or other taxable disposition of shares (including a redemption, but only if the redemption would be treated as a sale or exchange rather than a distribution for United States federal income tax purposes) unless:

We believe we currently are not, and we do not anticipate becoming, a USRPHC for United States federal income tax purposes. Generally, a corporation is a USRPHC only if the fair market value of its United States real property interests (as defined in the Code) equals or exceeds 50% of the sum of the fair market value of its worldwide real property interests and its other assets used or held for use in a trade or business.

Sections 1471 through 1474 of the Code (commonly referred to as the Foreign Account Tax Compliance Act, or FATCA) impose a 30% withholding tax on dividends paid on our shares to, and (subject to the proposed Treasury Regulations discussed below) the gross proceeds derived from the sale or other disposition of our shares by, a foreign entity if the foreign entity is:

Withholding under FATCA generally applies to payments of dividends on our shares and, after December 31, 2018, to payments of gross proceeds from a sale or other disposition of our shares. Withholding agents may, however, rely on recently proposed U.S. Treasury Regulations that would no longer require FATCA withholding on payments of gross proceeds. A withholding agent such as a broker, and not Omeros Corporation, will determine whether or not to implement gross proceeds FATCA withholding after December 31, 2018.

If a dividend payment is subject to withholding both under FATCA and the withholding tax rules discussed above under "—Dividends," the withholding under FATCA may be credited against, and therefore reduce, such other withholding tax. Holders of shares should consult their own tax advisors regarding these requirements and whether they may be relevant to their ownership and disposition of the shares.

Under certain circumstances, a non-U.S. holder will be eligible for refunds or credits of withholding taxes imposed under FATCA by filing a United States federal income tax return.

Prospective investors should consult their tax advisors regarding the effect of FATCA on their ownership and disposition of our shares.

Except as described below, a non-U.S. holder generally will be exempt from backup withholding and information reporting requirements with respect to dividend payments and the payment of the proceeds from the sale of our shares effected at a United States office of a broker, as long as the payor or broker does not have actual knowledge or reason to know that the holder is a United States person and has furnished to the payor or broker:

However, we must report annually to the Internal Revenue Service and to non-U.S. holders the amount of dividends paid to them and the tax withheld with respect to such dividends, regardless of whether withholding was required. Copies of the information returns reporting such dividends and withholding may also be made available to the tax authorities in the country in which the respective non-U.S. holder resides under the provisions of an applicable income tax treaty.

Payment of the proceeds from the sale of our common shares effected at a foreign office of a broker generally will not be subject to information reporting or backup withholding. However, a sale of our shares by a non-U.S. holder that is effected at a foreign office of a broker will be subject to information reporting and backup withholding if:

unless the broker does not have actual knowledge or reason to know that the holder is a United States person and the documentation requirements described above are met or the non-U.S. holder otherwise establishes an exemption.

In addition, a sale of shares will be subject to information reporting if it is effected at a foreign office of a broker that is:

unless the broker does not have actual knowledge or reason to know that the holder is a United States person and the documentation requirements described above are met or an exemption is otherwise established. Backup withholding will apply if the sale is subject to information reporting and the broker has actual knowledge that the holder is a United States person.

Backup withholding is not an additional tax. A non-U.S. holder generally may obtain a refund of any amounts withheld under the backup withholding rules that exceed the non-U.S. holder's income tax liability by timely filing a refund claim with the Internal Revenue Service.

The estates of nonresident alien decedents generally are subject to United States federal estate tax on property with a United States situs. Because we are a United States corporation, our shares will be United States situs property and therefore will be included in the taxable estate of a nonresident alien decedent at the time of the decedent's death, unless an applicable estate tax treaty between the United States and the decedent's country of residence provides otherwise. An estate tax credit is available to reduce the net tax liability of a nonresident alien's estate, but the estate tax credit for a nonresident alien is generally much smaller than the applicable credit for computing the estate tax of a United States resident. Nonresident aliens should consult their personal tax advisors regarding the United States federal estate tax consequences of owning our shares.

We have entered into an underwriting agreement, dated December 5, 2019, with Cantor Fitzgerald & Co. with respect to the common stock being offered hereby. Subject to the terms and conditions of the underwriting agreement, the underwriter has agreed to purchase from us 3,816,793 shares of our common stock.

The underwriting agreement provides that the obligation of the underwriter is subject to certain conditions precedent and that the underwriter has agreed to purchase all of the shares of common stock sold under the underwriting agreement if any of these shares are purchased, other than those shares of common stock covered by the option to purchase additional shares of common stock described below.

We have agreed to indemnify the underwriter against specified liabilities, including liabilities under the Securities Act, and to contribute to payments the underwriter may be required to make in respect thereof.

The underwriter is offering the shares of common stock, subject to prior sale, when, as and if issued to and accepted by it, subject to approval of legal matters by its counsel and other conditions specified in the underwriting agreement. The underwriter reserves the right to withdraw, cancel or modify offers to the public and to reject orders in whole or in part.

Underwriting fees and discount; offering expenses. The underwriter has advised us that it initially proposes to offer the shares of common stock, including the option to purchase an additional 572,518 shares, if applicable, to the public at the public offering price set forth on the cover page of this prospectus supplement and to certain dealers, which may include the underwriter. After the offering, the underwriter may change the offering price and other selling terms. The following table shows the public offering price, underwriting fees and discount and proceeds to us, before expenses, from the sale of our common stock. These amounts are shown assuming both no exercise and full exercise by Cantor Fitzgerald & Co. of its option to purchase 572,518 additional shares of common stock.

We estimate that the total expenses of the offering (which includes approximately $7,000 that we have agreed to reimburse the underwriter for out-of-pocket expenses incurred in connection with this offering), excluding the underwriting discount and commissions, will be approximately $275,000 and are payable by us.

Option to purchase additional shares of common stock. We have granted Cantor Fitzgerald & Co. an option to purchase from time to time, in whole or in part, up to an aggregate of 572,518 additional shares of our common stock at the price set forth on the cover page of this prospectus supplement. The option is exercisable for a period of 30 days. If the underwriter exercises the option in full, the total proceeds to us, before expenses, will be approximately $54.5 million.

Discretionary accounts. The underwriter does not intend to confirm sales of shares of common stock to any accounts over which it has discretionary authority.

Market Making, Stabilization and Other Transactions. The underwriter may make a market in the common stock as permitted by applicable laws and regulations. However, the underwriter is not obligated to do so and may discontinue any market-making activities at any time without notice in its sole discretion. Accordingly, no assurance can be given as to the liquidity of the trading market for the

common stock, that you will be able to sell any of the common stock held by you at a particular time or that the prices that you receive when you sell will be favorable.

The underwriter, pursuant to Regulation M under the Securities Exchange Act of 1934, as amended, may engage in short sale transactions, stabilizing transactions, syndicate covering transactions or the imposition of penalty bids in connection with this offering. These activities may have the effect of stabilizing or maintaining the market price of the common stock at a level above that which might otherwise prevail in the open market. Establishing short sales positions may involve either "covered" short sales or "naked" short sales.

"Covered" short sales are sales made in an amount not greater than the underwriter's option to purchase additional shares of our common stock in this offering. The underwriter may close out any covered short position by either exercising its option to purchase additional shares of our common stock or purchasing shares of our common stock in the open market. In determining the source of shares to close out the covered short position, the underwriter will consider, among other things, the price of shares available for purchase in the open market as compared to the price at which it may purchase shares through the option to purchase additional shares.

"Naked" short sales are sales in excess of the option to purchase additional shares of our common stock. The underwriter must close out any naked short position by purchasing shares in the open market. A naked short position is more likely to be created if the underwriter is concerned that there may be downward pressure on the price of the shares of our common stock in the open market after pricing that could adversely affect investors who purchase in this offering.

A stabilizing bid is a bid for the purchase of shares of common stock on behalf of the underwriter for the purpose of fixing or maintaining the price of the common stock. A syndicate covering transaction is the bid for or the purchase of shares of common stock on behalf of the underwriter to reduce a short position incurred by the underwriter in connection with the offering. Similar to other purchase transactions, the underwriter's purchases to cover the syndicate short sales may have the effect of raising or maintaining the market price of our common stock or preventing or retarding a decline in the market price of our common stock. As a result, the price of our common stock may be higher than the price that might otherwise exist in the open market. A penalty bid is an arrangement permitting the underwriter to reclaim the selling concession otherwise accruing to a syndicate member in connection with the offering if the common stock originally sold by such syndicate member are purchased in a syndicate covering transaction and therefore have not been effectively placed by such syndicate member.

Neither we, nor the underwriter make any representation or prediction as to the direction or magnitude of any effect that the transactions described above may have on the price of our common stock. The underwriter is not obligated to engage in these activities and, if commenced, may end any of these activities at any time.

Passive Market Making. The underwriter may also engage in passive market making transactions in our common stock on The Nasdaq Global Market in accordance with Rule 103 of Regulation M during a period before the commencement of offers or sales of shares of our common stock in this offering and extending through the completion of distribution. A passive market maker must display its bid at a price not in excess of the highest independent bid of that security. However, if all independent bids are lowered below the passive market maker's bid, that bid must then be lowered when specified purchase limits are exceeded. Passive market making may cause the price of our common stock to be higher than the price that otherwise would exist in the open market in the absence of those transactions. The underwriter is not required to engage in passive market making and, if commenced, may end passive market making activities at any time.

Lock-up agreements. Pursuant to certain "lock-up" agreements, we and our executive officers and directors, have agreed, subject to certain exceptions, not to offer, sell, assign, transfer, pledge, contract to sell, or otherwise dispose of or announce the intention to otherwise dispose of, or enter into any swap, hedge or similar agreement or arrangement that transfers, in whole or in part, the economic consequence of ownership of, directly or indirectly, or make any demand or request or exercise any right with respect to the registration of, or file with the SEC a registration statement under the Securities Act relating to, any common stock or securities convertible into or exchangeable or exercisable for any common stock without the prior written consent of the underwriter, for a period of 60 days after the date of the underwriting agreement.

This lock-up provision applies to common stock and to securities convertible into or exercisable or exchangeable for common stock. It also applies to common stock owned now or acquired later by the person executing the agreement or for which the person executing the agreement later acquires the power of disposition. The exceptions to the lock-up for executive officers and directors are for: (a) the transfer of shares of common stock or any securities convertible into or exercisable or exchangeable for common stock (i) as a bona fide gift to any member of the immediate family of the executive officer or director or to a trust formed for the benefit of an immediate family member or (ii) by will or intestate succession, or (iii) as a bona fide gift to a charity, non-profit organization or educational institution; (b) the exercise of options or warrants to purchase our common stock (including exercise on a "cashless" basis to the extent permitted by the instruments representing such options or warrants); (c) the establishment or modification of a trading plan pursuant to Rule 10b5-1 under the Exchange Act for the transfer of shares of common stock; (d) a transfer or sale of any shares pursuant to an existing Rule 10b5-1 plan; and (e) the pledge, transfer or sale of any shares of common stock or other securities pursuant to certain existing loan or security agreements between the executive officer or director (or a member of their immediate family) and a bank or other financial institution; each of which is subject to certain conditions set forth in the lock-up agreements with the executive officers and directors. The exceptions to the lock-up for us are: (i) our sale of shares of common stock in this offering; (ii) the issuance of restricted common stock or options to acquire common stock pursuant to our benefit plans, qualified equity incentive plans or other compensation plans; and (iii) the issuance of common stock in connection with a debt financing or an equity line transaction more than 30 days after the date of the underwriting agreement. In addition, under the lock-up we are permitted to enter into an agreement to sell shares of common stock in an "at the market offering" as defined in Rule 415(a)(4) under the Securities Act and to make related filings with the SEC, provided that we do not sell shares of common stock under the agreement during the lock-up period.

Canada. This prospectus supplement constitutes an "exempt offering document" as defined in and for the purposes of applicable Canadian securities laws. No prospectus has been filed with any securities commission or similar regulatory authority in Canada in connection with the offer and sale of the common stock. No securities commission or similar regulatory authority in Canada has reviewed or in any way passed upon this prospectus supplement or on the merits of the common stock and any representation to the contrary is an offence.

Canadian investors are advised that this prospectus supplement has been prepared in reliance on section 3A.3 of National Instrument 33-105 Underwriting Conflicts ("NI 33-105"). Pursuant to section 3A.3 of NI 33-105, this prospectus supplement is exempt from the requirement that the Company and the underwriter(s) provide investors with certain conflicts of interest disclosure pertaining to "connected issuer" and/or "related issuer" relationships that may exist between the Company and the underwriter(s) as would otherwise be required pursuant to subsection 2.1(1) of NI 33-105.

The offer and sale of the common stock in Canada is being made on a private placement basis only and is exempt from the requirement that the Company prepares and files a prospectus under applicable Canadian securities laws. Any resale of the common stock acquired by a Canadian investor in this offering must be made in accordance with applicable Canadian securities laws, which may vary depending on the relevant jurisdiction, and which may require resales to be made in accordance with Canadian prospectus requirements, pursuant to a statutory exemption from the prospectus requirements, in a transaction exempt from the prospectus requirements or otherwise under a discretionary exemption from the prospectus requirements granted by the applicable local Canadian securities regulatory authority. These resale restrictions may under certain circumstances apply to resales of the common stock outside of Canada.

Each Canadian investor who purchases the common stock will be deemed to have represented to the Company and the underwriter(s) that the investor (i) is purchasing the common stock as principal, or is deemed to be purchasing as principal in accordance with applicable Canadian securities laws, for investment only and not with a view to resale or redistribution; (ii) is an "accredited investor" as such term is defined in section 1.1 of National Instrument 45-106 Prospectus Exemptions ("NI 45-106") or, in Ontario, as such term is defined in section 73.3(1) of the Securities Act (Ontario); and (iii) is a "permitted client" as such term is defined in section 1.1 of National Instrument 31-103 Registration Requirements, Exemptions and Ongoing Registrant Obligations.

Any discussion of taxation and related matters contained in this prospectus supplement does not purport to be a comprehensive description of all of the tax considerations that may be relevant to a Canadian investor when deciding to purchase the common stock and, in particular, does not address any Canadian tax considerations. No representation or warranty is hereby made as to the tax consequences to a resident, or deemed resident, of Canada of an investment in the common stock or with respect to the eligibility of the common stock for investment by such investor under relevant Canadian federal and provincial legislation and regulations.

Securities legislation in certain of the Canadian jurisdictions provides certain purchasers of securities pursuant to an offering memorandum (such as this prospectus supplement), including where the distribution involves an "eligible foreign security" as such term is defined in Ontario Securities Commission Rule 45-501 Ontario Prospectus and Registration Exemptions and in Multilateral Instrument 45-107 Listing Representation and Statutory Rights of Action Disclosure Exemptions, as applicable, with a remedy for damages or rescission, or both, in addition to any other rights they may have at law, where the offering memorandum, or other offering document that constitutes an offering memorandum, and any amendment thereto, contains a "misrepresentation" as defined under applicable Canadian securities laws. These remedies, or notice with respect to these remedies, must be exercised or delivered, as the case may be, by the purchaser within the time limits prescribed under, and are subject to limitations and defences under, applicable Canadian securities legislation. In addition, these remedies are in addition to and without derogation from any other right or remedy available at law to the investor.

Upon receipt of this document, each Canadian investor hereby confirms that it has expressly requested that all documents evidencing or relating in any way to the sale of the securities described herein (including for greater certainty any purchase confirmation or any notice) be drawn up in the English language only. Par la réception de ce document, chaque investisseur Canadien confirme par les présentes qu'il a expressément exigé que tous les documents faisant foi ou se rapportant de quelque manière que ce soit à la vente des valeurs mobilières décrites aux présentes (incluant, pour plus de certitude, toute confirmation d'achat ou tout avis) soient rédigés en anglais seulement.

Australia. This prospectus is not a disclosure document for the purposes of Australia's Corporations Act 2001 (Cth) of Australia, or Corporations Act, has not been lodged with the Australian Securities & Investments Commission and is only directed to the categories of exempt persons set out below. Accordingly, if you receive this prospectus in Australia:

To the extent that you are unable to confirm or warrant that you are an exempt sophisticated investor or professional investor under the Corporations Act any offer made to you under this prospectus is void and incapable of acceptance.

You warrant and agree that you will not offer any of the shares issued to you pursuant to this prospectus for resale in Australia within 12 months of those securities being issued unless any such resale offer is exempt from the requirement to issue a disclosure document under section 708 of the Corporations Act.

European Economic Area. In relation to each Member State of the European Economic Area, no offer of any securities which are the subject of the offering contemplated by this prospectus has been or will be made to the public in that Member State other than any offer where a prospectus has been or will be published in relation to such securities that has been approved by the competent authority in that Member State or, where appropriate, approved in another Member State and notified to the relevant competent authority in that Member State, all in accordance with the Prospectus Regulation, except that an offer of such securities may be made to the public in that Member State at any time under the following exemptions under the Prospectus Regulation:

provided that no such offer of securities shall require the Company or the underwriter to publish a prospectus pursuant to Article 3 of the Prospectus Regulation or supplement a prospectus pursuant to Article 23 of the Prospectus Regulation.

For the purposes of this provision, the expression an "offer to the public" in relation to any securities in any Member State means the communication in any form and by any means of sufficient information on the terms of the offer and the securities to be offered so as to enable an investor to decide to purchase or subscribe for the securities, and the expression "Prospectus Regulation" means Regulation (EU) 2017/1129.

Hong Kong. No securities have been offered or sold, and no securities may be offered or sold, in Hong Kong, by means of any document, other than to persons whose ordinary business is to buy or sell shares or debentures, whether as principal or agent; or to "professional investors" as defined in the Securities and Futures Ordinance (Cap. 571) of Hong Kong and any rules made under that Ordinance; or in other circumstances which do not result in the document being a "prospectus" as defined in the Companies Ordinance (Cap. 32) of Hong Kong or which do not constitute an offer to the public within the meaning of the Companies Ordinance (Cap.32) of Hong Kong. No document, invitation or advertisement relating to the securities has been issued or may be issued or may be in the possession of any person for the purpose of issue (in each case whether in Hong Kong or elsewhere), which is directed at, or the contents of which are likely to be accessed or read by, the public of Hong Kong (except if permitted under the securities laws of Hong Kong) other than with respect to securities which are or are intended to be disposed of only to persons outside Hong Kong or only to "professional investors" as defined in the Securities and Futures Ordinance (Cap. 571) of Hong Kong and any rules made under that Ordinance.

This prospectus has not been registered with the Registrar of Companies in Hong Kong. Accordingly, this prospectus may not be issued, circulated or distributed in Hong Kong, and the securities may not be offered for subscription to members of the public in Hong Kong. Each person acquiring the securities will be required, and is deemed by the acquisition of the securities, to confirm that he is aware of the restriction on offers of the securities described in this prospectus and the relevant offering documents and that he is not acquiring, and has not been offered any securities in circumstances that contravene any such restrictions.

Japan. The offering has not been and will not be registered under the Financial Instruments and Exchange Law of Japan (Law No. 25 of 1948 of Japan, as amended), or FIEL, and the Initial Purchaser will not offer or sell any securities, directly or indirectly, in Japan or to, or for the benefit of, any resident of Japan (which term as used herein means, unless otherwise provided herein, any person resident in Japan, including any corporation or other entity organized under the laws of Japan), or to others for re-offering or resale, directly or indirectly, in Japan or to a resident of Japan, except pursuant to an exemption from the registration requirements of, and otherwise in compliance with, the FIEL and any other applicable laws, regulations and ministerial guidelines of Japan.

Singapore. This prospectus has not been and will not be lodged or registered with the Monetary Authority of Singapore. Accordingly, this prospectus and any other document or material in connection with the offer or sale, or the invitation for subscription or purchase of the securities may not be issued, circulated or distributed, nor may the securities be offered or sold, or be made the subject of an invitation for subscription or purchase, whether directly or indirectly, to the public or any member of the public in Singapore other than (i) to an institutional investor under Section 274 of the Securities and Futures Act, Chapter 289 of Singapore, or the SFA, (ii) to a relevant person as defined under Section 275(2), or any person pursuant to Section 275(1A) of the SFA, and in accordance with the conditions, specified in Section 275 of the SFA, or (iii) otherwise pursuant to, and in accordance with the conditions of any other applicable provision of the SFA.

Where the securities are subscribed or purchased under Section 275 of the SFA by a relevant person which is:

shares, debentures and units of shares and debentures of that corporation or the beneficiaries' rights and interest in that trust shall not be transferable for six months after that corporation or that trust has acquired the Offer Shares under Section 275 of the SFA except:

Switzerland. The securities may not be publicly offered in Switzerland and will not be listed on the SIX Swiss Exchange, or SIX, or on any other stock exchange or regulated trading facility in Switzerland. This prospectus has been prepared without regard to the disclosure standards for issuance prospectuses under art. 652a or art. 1156 of the Swiss Code of Obligations or the disclosure standards for listing prospectuses under art. 27 ff. of the SIX Listing Rules or the listing rules of any other stock exchange or regulated trading facility in Switzerland. Neither this prospectus nor any other offering or marketing material relating to the securities or the offering may be publicly distributed or otherwise made publicly available in Switzerland.

Neither this prospectus nor any other offering or marketing material relating to the offering, the Company or the securities have been or will be filed with or approved by any Swiss regulatory authority. In particular, this prospectus will not be filed with, and the offer of securities will not be supervised by, the Swiss Financial Market Supervisory Authority FINMA, or FINMA, and the offer of securities has not been and will not be authorized under the Swiss Federal Act on Collective Investment Schemes, or CISA. The investor protection afforded to acquirers of interests in collective investment schemes under the CISA does not extend to acquirers of securities.

Israel. This document does not constitute a prospectus under the Israeli Securities Law, 5728-1968, or the Securities Law, and has not been filed with or approved by the Israel Securities Authority. In the State of Israel, this document is being distributed only to, and is directed only at, and any offer of the shares is directed only at, investors listed in the first addendum, or the Addendum, to the Israeli Securities Law, consisting primarily of joint investment in trust funds, provident funds, insurance companies, banks, portfolio managers, investment advisors, members of the Tel Aviv Stock Exchange, underwriters, venture capital funds, entities with equity in excess of NIS 50 million and "qualified individuals", each as defined in the Addendum (as it may be amended from time to time), collectively referred to as qualified investors (in each case purchasing for their own account or, where permitted under the Addendum, for the accounts of their clients who are investors listed in the Addendum). Qualified investors will be required to submit written confirmation that they fall within the scope of the Addendum, are aware of the meaning of same and agree to it.

United Kingdom. This prospectus is only being distributed to, and is only directed at, persons in the United Kingdom that are qualified investors (as defined in the Prospectus Regulation) that are also (i) investment professionals falling within Article 19(5) of the Financial Services and Markets Act 2000 (Financial Promotion) Order 2005, as amended, referred to herein as the "Order", and/or (ii) high net worth entities falling within Article 49(2)(a) to (d) of the Order and other persons to whom it may lawfully be communicated or caused to be communicated. Each such person is referred to herein as a "Relevant Person".

This prospectus and its contents are confidential and should not be distributed, published or reproduced (in whole or in part) or disclosed by recipients to any other persons in the United Kingdom. Any person in the United Kingdom that is not a Relevant Person should not act or rely on this document or any of its contents.

Any invitation or inducement to engage in investment activity (within the meaning of Section 21 of the Financial Services and Markets Act 2000 (the "FSMA") may only be communicated or caused to be communicated in connection with the issue or sale of the securities in circumstances in which Section 21(1) of the FSMA does not apply. All applicable provisions of the FSMA must be complied with in respect of anything done by any person in relation to the securities in, from or otherwise involving the United Kingdom.

Electronic offer, sale and distribution of shares of common stock. A prospectus in electronic format may be made available on the websites maintained by the underwriter or selling group members, if any, participating in this offering and the underwriter participating in this offering may distribute prospectuses electronically. Other than the prospectus in electronic format, the information on these websites is not part of this prospectus supplement, the accompanying prospectus or the registration statement of which the accompanying prospectus forms a part, has not been approved or endorsed by us or the underwriter in its capacity as underwriter, and should not be relied upon by investors.

The underwriter and certain of its affiliates are full service financial institutions engaged in a wide range of activities for their own accounts and the accounts of customers, which may include, among other things, corporate finance, mergers and acquisitions, merchant banking, equity and fixed income sales, trading and research, derivatives, foreign exchange, futures, asset management, custody, clearance and securities lending. The underwriter and certain of its affiliates have, from time to time, performed, and may in the future perform, various investment banking and financial advisory services for us and our affiliates, for which it received or will receive customary fees and expenses.

Listing on The Nasdaq Global Market. Our common stock is traded on The Nasdaq Global Market under the symbol "OMER." The transfer agent for our common stock is Computershare Inc.

The validity of the common stock offered hereby will be passed upon by Keller Rohrback L.L.P., Seattle, Washington. Certain other legal matters in connection with this offering will be passed upon by Covington & Burling LLP, Washington, D.C., Peter B. Cancelmo, Vice President and General Counsel of Omeros, and Tina Quinton, Vice President, Patents of Omeros. As of December 4, 2019, Mr. Cancelmo held 200 shares of our common stock and Mr. Cancelmo and Ms. Quinton held options under our equity incentive plans to purchase up to 40,000 and 143,906 shares of our common stock, respectively, and each was eligible to receive additional equity awards under our 2017 Omnibus Incentive Compensation Plan. Duane Morris LLP, New York, New York is counsel to the underwriter in connection with this offering.

Our consolidated financial statements appearing in our Annual Report (Form 10-K) for the year ended December 31, 2018, and the effectiveness of Omeros' internal control over financial reporting as of December 31, 2018, have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in its reports thereon (which contain an explanatory paragraph describing conditions that raise substantial doubt about our ability to continue as a going concern as described in Note 1 to the consolidated financial statements) included therein, and incorporated herein by reference. Such consolidated financial statements are incorporated herein by reference in reliance upon such reports given on the authority of such firm as experts in accounting and auditing.

We file annual, quarterly and current reports, proxy statements and other information with the SEC. Our SEC filings are available to the public over the Internet at the SEC's website at www.sec.gov. Our Annual Report on Form 10-K, Quarterly Reports on Form 10-Q and Current Reports on Form 8-K, including any amendments to those reports, and other information that we file with or furnish to the SEC pursuant to Section 13(a) or 15(d) of the Exchange Act can also be accessed free of charge by linking directly from our website at www.omeros.com under the "Investors and News—Financial Information—SEC Filings" caption to the SEC's Edgar Database. These filings will be available as soon as reasonably practicable after we electronically file such material with, or furnish it to, the SEC.

We have filed with the SEC a registration statement under the Securities Act relating to the common stock in this offering. The registration statement, including the attached exhibits, contains additional relevant information about us and the securities. This prospectus supplement and the accompanying prospectus do not contain all of the information set forth in the registration statement. The registration statement and the documents referred to below under "Incorporation by Reference" are also available on our website, www.omeros.com. We have not incorporated by reference into this prospectus supplement the information on, or that can be accessed through, our website, and you should not consider it to be a part of this prospectus supplement.

The SEC allows us to "incorporate by reference" the information we file with it, which means that we can disclose important information to you by referring you to those documents. The information incorporated by reference is an important part of this prospectus supplement and the accompanying prospectus, and information that we file later with the SEC will automatically update and supersede this information. The following documents that we have previously filed with the SEC (File No. 001-34475) are incorporated by reference herein:

All reports and other documents that we subsequently file pursuant to Section 13(a), 13(c), 14 or 15(d) of the Exchange Act prior to the termination of this offering, but excluding any information furnished to, rather than filed with, the SEC, will also be incorporated by reference into this prospectus supplement and the accompanying prospectus and deemed to be part of this prospectus supplement and the accompanying prospectus from the time of the filing of such reports and documents.

This prospectus supplement and the accompanying prospectus as further supplemented may contain information that updates, modifies or is contrary to information herein or in one or more of the documents incorporated by reference in this prospectus supplement or the accompanying prospectus. You should rely only on the information incorporated by reference or provided in this prospectus supplement and accompanying prospectus. We have not authorized anyone else to provide you with different information. You should not assume that the information in this prospectus supplement and the accompanying prospectus is accurate as of any date other than the date of this prospectus supplement, the date of the accompanying prospectus or the date of the documents incorporated by reference in this prospectus supplement and the accompanying prospectus, respectively.

We will provide without charge to each person to whom this prospectus supplement and accompanying prospectus is delivered, upon written or oral request, a copy of any or all documents that are incorporated by reference into this prospectus supplement and accompanying prospectus, but not delivered with the prospectus supplement and accompanying prospectus. We will not provide exhibits to such documents unless such exhibits are specifically incorporated by reference into the documents that this prospectus supplement or accompanying prospectus incorporates. You should direct written requests to: Omeros Corporation, Attn: Legal Department, 201 Elliott Avenue West, Seattle, Washington 98119, or you may call us at (206) 676-5000.

Common Stock
Preferred Stock
Debt Securities
Depositary Shares
Warrants
Subscription Rights
Units

From time to time, we may offer and sell the securities described in this prospectus separately or together in any combination, in one or more classes or series, in amounts, at prices and on terms that we will determine at the time of the offering.

This prospectus may not be used to sell securities unless accompanied by a prospectus supplement, which will describe the method and specific terms of the offering, including the amount, price and terms of the applicable offered securities. You should carefully read this prospectus, the information incorporated by reference in this prospectus, the prospectus supplement, including any information incorporated by reference in such prospectus supplement, and any free writing prospectus before you purchase any of the securities offered hereby.

These securities may be offered and sold in the same offering or in separate offerings, to or through underwriters, dealers or agents or directly to purchasers. The names of any underwriters, dealers or agents involved in the sale of our securities, their compensation and any options held by them to purchase additional securities will be described in the applicable prospectus supplement. See "Plan of Distribution."

Our common stock is listed on The Nasdaq Global Market under the symbol "OMER." On December 3, 2019, the closing price of our common stock was $14.37 per share.

Investing in our securities involves significant risks. Before buying our securities, you should carefully consider the risks described under the caption "Risk Factors" beginning on page 3 of this prospectus, in the documents incorporated by reference into this prospectus, and in the applicable prospectus supplement.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

This prospectus is part of a registration statement on Form S-3 that we filed with the United States Securities and Exchange Commission (the "SEC") using an automatic shelf registration process. Under this shelf process, we may, from time to time, offer or sell any combination of the securities described in this prospectus in one or more offerings.

This prospectus only provides you with a general description of the securities that we may offer. Each time we sell securities pursuant to this prospectus, we will describe in a prospectus supplement, which will be delivered with this prospectus, specific information about the offering and the terms of the particular securities offered. The prospectus supplement may also add, update or change the information contained in this prospectus. Before purchasing any securities, you should carefully read both this prospectus and the accompanying prospectus supplement and any free writing prospectus prepared by or on behalf of us, together with the additional information described under "Where You Can Find More Information."

This prospectus does not contain all of the information included in the registration statement. For a more complete understanding of the offering of the securities, you should refer to the registration statement, including its exhibits. Those exhibits may be filed with the registration statement or may be incorporated by reference to earlier SEC filings listed in the registration statement or in subsequent filings that we may make under the Securities Exchange Act of 1934 (the "Exchange Act").

We have not authorized anyone else to provide you with information that is different or in addition to that contained or incorporated by reference in this prospectus supplement, the accompanying prospectus or any free writing prospectus prepared by us or on our behalf. We do not take any responsibility for, and can provide no assurance to the reliability of, any information that others may give. We are not making an offer to sell or soliciting an offer to buy our securities under any circumstance in any jurisdiction where the offer or solicitation is not permitted. You should assume that the information contained in this prospectus, any prospectus supplement or any free writing prospectus prepared by us or on our behalf is accurate only as of the date of the respective document in which the information appears, and that any information in documents that we have incorporated by reference is accurate only as of the date of the document incorporated by reference, regardless of the time of delivery of this prospectus or any prospectus supplement or any sale of a security. Our business, financial condition, results of operations and prospects may have changed since those dates.

This prospectus, any prospectus supplement and the information incorporated herein and therein by reference includes trademarks, service marks and trade names owned by us or other companies. All trademarks, service marks and trade names included or incorporated by reference into this prospectus supplement or the accompanying prospectus are the property of their respective owners.

Unless the context indicates otherwise, in this prospectus supplement and the accompanying prospectus the terms "Omeros," "we," "us," and "our" refer to Omeros Corporation, a Washington corporation, and its subsidiaries on a consolidated basis.

This prospectus, each prospectus supplement and the information incorporated by reference in this prospectus and each prospectus supplement contain forward-looking statements within the meaning of Section 27A of the Securities Act of 1933 (the "Securities Act"), and Section 21E of the Exchange Act, which are subject to the "safe harbor" created by those sections for such statements. Forward-looking statements are based on our management's beliefs and assumptions and on information currently available to our management. All statements other than statements of historical fact are "forward-looking statements." Terms such as "anticipate," "believe," "could," "estimate," "expect," "goal," "intend," "may," "plan," "potential," "predict," "project," "should," "target," "will," "would," and similar expressions and variations thereof are intended to identify forward-looking statements, but these terms are not the exclusive means of identifying such statements. Those statements appear in this prospectus, any accompanying prospectus supplement and the documents incorporated herein and therein by reference, particularly in the sections entitled "Prospectus Summary" and "Risk Factors," and include statements regarding the intent, belief or current expectations of us and our management that are subject to known and unknown risks, uncertainties and assumptions. Examples of forward-looking statements include, but are not limited to, statements regarding:

Our actual results could differ materially from those anticipated in these forward-looking statements for many reasons, including the risks, uncertainties and other factors described in our filings with the SEC that are incorporated herein by reference. Given these risks, uncertainties and other factors, actual results or anticipated developments may not be realized or, even if substantially realized, may not have the expected consequences to or effects on our company, business or operations. Accordingly, you should not place undue reliance on these forward-looking statements, which represent our estimates and assumptions only as of the date they are made. The events and circumstances reflected in the forward-looking statements may not be achieved or occur and actual results may materially differ from those projected in the forward-looking statements. Except as required by applicable law, we assume no obligation to update or revise any forward-looking statements contained in this prospectus, the accompanying prospectus supplement or any information incorporated by reference herein or therein, whether as a result of any new information, future events or otherwise.

Clinical Findings in Pivotal Trial of HSCT-TMA Patients Treated with Narsoplimab

and neurological findings. Patients in the trial had a high expected mortality rate, with 93% of them having multiple risk factors. No signal of any serious safety risk was observed with narsoplimab in the trial. The most common adverse events observed in the trial were nausea, vomiting, diarrhea, hypokalemia, neutropenia and fever, which are all common in stem-cell transplant patients. 21% of patients died during the trial due to causes common in stem cell transplant, with no additional patients discontinuing for adverse events. The data from the patients who died were not excluded from any analyses.

We were incorporated in the State of Washington in 1994. Our principal executive offices are located at 201 Elliott Avenue West, Seattle, Washington 98119, and our telephone number is (206) 676-5000.

We file annual, quarterly and current reports, proxy statements and other information with the SEC. Our SEC filings are available to the public over the Internet at the SEC's website at www.sec.gov. Our Annual Report on Form 10-K, Quarterly Reports on Form 10-Q, and Current Reports on Form 8-K, including any amendments to those reports, and other information that we file with or furnish to the SEC pursuant to Section 13(a) 13(c), 14 or 15(d) of the Exchange Act can also be accessed free of charge on our website at www.omeros.com under "Investors and News—Financial Information—SEC Filings." These filings will be available as soon as reasonably practicable after we electronically file such material with, or furnish it to, the SEC.

We have filed with the SEC a registration statement under the Securities Act relating to the offering of these securities. The registration statement, including the attached exhibits, contains additional relevant information about us and the securities. This prospectus does not contain all of the information set forth in the registration statement. You can obtain a copy of the registration statement from the SEC's website at the address listed above. The registration statement and the documents

referred to below under "Incorporation by Reference" are also available on our website, www.omeros.com. We have not incorporated by reference into this prospectus the information on, or that can be accessed through, our website, and you should not consider it to be a part of this prospectus.

The SEC allows us to "incorporate by reference" the information we file with it, which means that we can disclose important information to you by referring you to those documents. The information incorporated by reference is an important part of this prospectus, and information that we file later with the SEC will automatically update and supersede this information. The following documents that we have previously filed with the SEC (File No. 001-34475) are incorporated by reference herein:

All reports and other documents that we subsequently file pursuant to Section 13(a), 13(c), 14 or 15(d) of the Exchange Act prior to the completion of the sales of the securities offered hereby but excluding any information furnished to, rather than filed with, the SEC (unless expressly incorporated by reference herein), will also be incorporated by reference into this prospectus and deemed to be part of this prospectus from the date of the filing of such reports and documents.

This prospectus, as supplemented, may contain information that updates, modifies or is contrary to information in one or more of the documents incorporated by reference in this prospectus. You should rely only on the information incorporated by reference or provided in this prospectus, the accompanying prospectus supplement or any free writing prospectus that we have prepared that relates to a particular offering. We have not authorized anyone else to provide you with different or additional information. You should not assume that the information in this prospectus is accurate as of any date other than the date of this prospectus or the date of the documents incorporated by reference in this prospectus.

We will provide without charge to each person, including any beneficial owner, to whom this prospectus is delivered, upon written or oral request, a copy of any or all documents that are incorporated by reference into this prospectus, but not delivered with the prospectus, other than exhibits to such documents unless such exhibits are specifically incorporated by reference into the documents that this prospectus incorporates. You should direct written requests to: Omeros Corporation, Attn: Legal Department, 201 Elliott Avenue West, Seattle, Washington 98119, or you may call us at (206) 676-5000.

An investment in our securities involves a high degree of risk. The prospectus supplement applicable to each offering of our securities will contain a discussion of the risks applicable to an investment in our securities. Before deciding whether to invest in our securities, you should consider

carefully the risks described under the heading "Risk Factors" in the applicable prospectus supplement and in any of our filings with the SEC that are incorporated by reference therein.

Unless otherwise indicated in the prospectus supplement, we intend to use the net proceeds from the sale of securities offered under this prospectus for general corporate purposes, including expenses related to funding research and development expenses for our narsoplimab programs and clinical trials, pre-clinical studies, manufacturing and other costs associated with advancing our product candidates toward Marketing Authorization Application, Biologic License Application and New Drug Application submissions. We may also use the net proceeds for working capital, the repayment of debt obligations, acquisitions or investments in businesses, products or technologies that are complementary to our own, and other capital expenditures. We will set forth in the prospectus supplement our intended use for the net proceeds received from the sale of any securities. Pending the use of such proceeds, we intend to invest the net proceeds in short-term, investment grade, interest-bearing instruments.

We may issue, separately or together with, or upon conversion, exercise or exchange of other securities, common stock, par value $0.01 per share, as set forth in the applicable prospectus supplement. The following summary of our common stock does not purport to be complete and is subject to, and is qualified in its entirety by reference to, our Amended and Restated Articles of Incorporation (the "Articles of Incorporation"), Amended and Restated Bylaws (the "Bylaws"), and applicable provisions of the Washington Business Corporation Act (the "WBCA"). Therefore, you should carefully consider the actual provisions of our Articles of Incorporation and Bylaws as well as relevant portions of the WBCA.

Our authorized capital stock consists of (i) 150,000,000 shares of common stock, par value $0.01 per share, and (ii) 20,000,000 shares of preferred stock, par value $0.01 per share. As of September 30, 2019, 49,525,474 shares of common stock were outstanding and no shares of preferred stock were outstanding. All outstanding shares of common stock are fully paid and nonassessable.

Subject to preferences that may be applicable to any outstanding shares of preferred stock, holders of common stock are entitled to receive ratably such dividends as may be declared by the board of directors out of funds legally available therefor.

The holders of our common stock are entitled to one vote per share on all matters to be voted on by the shareholders. Our Articles of Incorporation provide that shareholders are not entitled to cumulate votes in the election of directors.

Holders of common stock have no preemptive, conversion or subscription rights. There are no redemption or sinking fund provisions applicable to our common stock.

If we liquidate, dissolve or wind up, holders of our common stock are entitled to share ratably in all assets remaining after payment of liabilities and the liquidation preferences of any outstanding shares of preferred stock.

Our common stock is listed on The Nasdaq Global Market under the symbol "OMER." The transfer agent and registrar for our common stock is Computershare Inc. Its address is P.O. Box 505000, Louisville, Kentucky 40233-5000 and its telephone number is 1-866-282-4938.

As of September 30, 2019, we had outstanding warrants to purchase an aggregate of 243,115 shares of our common stock. The warrants have a weighted average exercise price of approximately $20.68 per share and will expire in the second quarter of 2023.

Anti-Takeover Effects of Washington Law and our Articles of Incorporation and Bylaws

Certain provisions of Washington law, our Articles of Incorporation and our Bylaws contain provisions that may delay, defer or discourage another party from acquiring control of us. These provisions, which are summarized below, are expected to discourage coercive takeover practices and inadequate takeover bids. These provisions are also designed, in part, to encourage persons seeking to acquire control of us to first negotiate with our board of directors. We believe that the benefits of increased protection of our potential ability to negotiate with an unfriendly or unsolicited acquiror outweigh the disadvantages of discouraging a proposal to acquire us because negotiation of these proposals could result in an improvement of their terms.

Chapter 23B.19 of the Washington Business Corporation Act, with limited exceptions, prohibits a "target corporation" from engaging in specified "significant business transactions" for a period of five years after the share acquisition by an "acquiring person", unless (a) the significant business transaction or the acquiring person's purchase of shares was approved by a majority of the members of the target corporation's board of directors prior to the acquiring person's share acquisition or (b) the significant business transaction was both approved by the majority of the members of the target corporation's board and authorized at a shareholder meeting by at least two-thirds of the outstanding voting shares (excluding the acquiring person's shares or shares over which the acquiring person has voting control) at or subsequent to the acquiring person's share acquisition. An "acquiring person" is defined as a person or group of persons that beneficially owns 10% or more of the voting securities of the target corporation. "Significant business transactions" include, among other transactions:

After the five-year period, "significant business transactions" are permitted, as long as they comply with the "fair price" provisions of the statute or are approved by a majority of the outstanding shares other than those of which the acquiring person has beneficial ownership. A corporation may not "opt out" of this statute. This statute could prohibit or delay the accomplishment of mergers or other takeover or change in control attempts with respect to us and, accordingly, may discourage attempts to acquire us.

Our Articles of Incorporation and Bylaws provide that shareholders can amend or repeal our bylaws only upon the affirmative vote of the holders of our voting stock.

As discussed below, our board of directors has the ability to issue preferred stock with voting or other rights or preferences that could impede the success of any attempt to effect a change of control. These and other provisions may have the effect of deferring hostile takeovers or delaying changes in control or management.

Limits on Ability of Shareholders to Act by Written Consent or Call a Special Meeting

Washington law limits the ability of shareholders of public companies from acting by written consent by requiring unanimous written consent for a shareholder action to be effective. This limit on the ability of our shareholders to act by less than unanimous written consent may lengthen the amount of time required to take shareholder actions. As a result, a holder controlling a majority of our capital stock who is unable to obtain unanimous written consent from all of our shareholders would not be able to amend our Bylaws or remove directors without holding a shareholders meeting.

In addition, our Articles of Incorporation provide that, unless otherwise required by law, special meetings of the shareholders may be called only by the chairman of the board, the chief executive officer, the president, or the board of directors acting pursuant to a resolution adopted by a majority of the board members. A shareholder may not call a special meeting, which may delay the ability of our shareholders to force consideration of a proposal or for holders controlling a majority of our capital stock to take any action, including the removal of directors.

Requirements for Advance Notification of Shareholder Nominations and Proposals

Our Bylaws establish advance notice procedures with respect to shareholder proposals and the nomination of candidates for election as directors, other than nominations made by or at the direction of the board of directors or a committee of the board of directors. The Bylaws do not give the board of directors the power to approve or disapprove shareholder nominations of candidates or proposals regarding business to be conducted at a special or annual meeting of the shareholders. However, our Bylaws may have the effect of precluding the conduct of certain business at a meeting if the proper procedures are not followed. These provisions may also discourage or deter a potential acquiror from conducting a solicitation of proxies to elect the acquiror's own slate of directors or otherwise attempting to obtain control of us.

Only our board of directors may determine the number of directors on our board and fix such number by resolution from time to time. Our Articles of Incorporation provide that vacancies and newly created seats on our board of directors may only be filled by the majority vote of the remaining members of our board of directors. The inability of our shareholders to determine the number of directors or to fill vacancies or newly created seats on our board of directors makes it more difficult to change the composition of our board of directors, but these provisions may promote a continuity of existing management.

Our directors may be removed only for cause by the affirmative vote of the holders of our voting stock at a meeting of shareholders called for such purpose.

Our board of directors is divided into three classes. The directors in each class will serve for a three-year term, with one class being elected each year by our shareholders. This system of electing directors may tend to discourage a third party from making a tender offer or otherwise attempting to obtain control of us, because it generally makes it more difficult for shareholders to replace a majority of the directors.

Our Articles of Incorporation provide that shareholders are not entitled to cumulate votes in the election of directors.

We may issue, separately or together with, or upon conversion, exercise or exchange of other securities, preferred stock, par value $0.01 per share, as set forth in the applicable prospectus supplement. The following summary does not purport to be complete and is subject to, and is qualified in its entirety by reference to, our Articles of Incorporation, Bylaws and applicable provisions of the WBCA. Therefore, you should carefully consider the actual provisions of our Articles of Incorporation and Bylaws as well as relevant portions of the WBCA.

Under the terms of our Articles of Incorporation, our board of directors is authorized, from time to time and without further shareholder action, to issue of up to 20,000,000 shares of preferred stock in one or more series, and to fix the designations, powers, preferences and rights, and the qualifications, limitations or restrictions applicable to the preferred stock. As of the date of this prospectus, no shares of preferred stock were outstanding and our board of directors has made no provision for the issuance of any series of preferred stock.

The rights, preferences, privileges and restrictions of the preferred stock of each series will be fixed by Articles of Amendment to the Articles of Incorporation relating to that series and will be described in the applicable prospectus supplement. Our board of directors has the discretion to fix the number of shares of any such series and the designation thereof, and to fix and amend the powers, preferences and rights, and the limitations or restrictions granted to or imposed upon any wholly unissued series of preferred stock, including the voting rights, dividend rights, dividend rate, conversion rights, rights and terms of redemption (including sinking fund provisions), redemption prices and liquidation preferences of any such series. Our board of directors has authority to decrease the number of shares of any series of preferred stock that is outstanding, but not below the number of shares of any such series then outstanding. In addition, any such reduction is subject to the powers, preferences and rights, and the qualifications, limitations and restrictions thereof stated in the Articles of Incorporation or the board resolution originally fixing the number of shares of such series.

The issuance of preferred stock will affect, and may adversely affect, the rights of holders of common stock. It is not possible to state the actual effect of the issuance of any shares of preferred stock on the rights of holders of common stock until our board of directors determines the specific rights, preferences and privileges attached to that series of preferred stock. The effects of issuing preferred stock could include one or more of the following:

The applicable prospectus supplement will specify the terms of the series of preferred stock being offered, including the following:

We have no present plans to issue any shares of preferred stock nor are any shares of our preferred stock presently outstanding. Preferred stock will be fully paid and nonassessable upon issuance.

Unless otherwise specified in the applicable prospectus supplement, the preferred stock will, with respect to distribution rights and rights upon liquidation, dissolution or winding up, rank (i) senior to our common stock and to any series of preferred stock which specifically provides that it will rank junior to the preferred stock being offered, (ii) junior to any series of preferred stock which specifically provides that it will rank senior to the preferred stock being offered and (iii) on parity with any other series of preferred stock.

Holders of preferred stock will have the dividend rights set forth in the applicable prospectus supplement. Dividends on any series of preferred stock, if cumulative, will be cumulative from and after the date set forth in the applicable prospectus supplement. Any restriction on the repurchase or redemption of shares of preferred stock while dividends on such shares are in arrears shall be set forth in the applicable prospectus supplement.

We will appoint a transfer agent and registrar for the preferred stock that will be set forth in the applicable prospectus supplement.

Refer to "Description of Common Stock—Anti-Takeover Effects of Washington Law and our Articles of Incorporation and Bylaws" for a discussion of provisions of the WBCA and our Articles of Incorporation and Bylaws that may have the effect of delaying, deferring or preventing a change of control.

We may issue, separately or together with, or upon conversion, exercise or exchange of other securities, debt securities, including debentures, notes, bonds and other evidences of indebtedness as set forth in the applicable prospectus supplement. The debt securities may be either secured or unsecured and will either be senior debt securities or subordinated debt securities. The debt securities will be issued under one or more separate indentures between us and a trustee to be specified in an accompanying prospectus supplement. Senior debt securities will be issued under a senior indenture and subordinated debt securities will be issued under a subordinated indenture. Together, the senior indenture and the subordinated indenture are called indentures in this description. This prospectus, together with the applicable prospectus supplement, will describe the terms of a particular series of debt securities that we may offer from time to time.

The following summary of the material provisions of the indentures and the debt securities does not purport to be complete and is subject to, and is qualified in its entirety by reference to, the provisions of the applicable indenture and certificates evidencing the applicable debt securities. Therefore, you should carefully consider the applicable indenture and the certificate evidencing the applicable debt security that is filed as an exhibit to the registration statement that includes this prospectus. Other specific terms of the applicable indenture and debt securities will be described in the applicable prospectus supplement. If any particular terms of the indenture or debt securities described in a prospectus supplement differ from any of the terms described below, then the terms described below will be deemed to have been superseded by that prospectus supplement. In this description of the debt securities, the words "Omeros Corporation," "we," "us" or "our" refer only to Omeros Corporation and not to our subsidiary, unless we otherwise expressly state or the context otherwise requires.

Debt securities may be issued in separate series without limitation as to aggregate principal amount. We may specify a maximum aggregate principal amount for the debt securities of any series.

We are not limited as to the amount of debt securities that we may issue under the indentures. Unless otherwise provided in a prospectus supplement, a series of debt securities may be reopened to issue additional debt securities of such series.

The prospectus supplement relating to a particular series of debt securities will set forth the material terms of the debt securities being offered, including:

If we denominate the purchase price of any of the debt securities in a foreign currency or currencies, or if the principal of or premium, if any, or interest on any series of debt securities is payable in a foreign currency or currencies, we will include in the applicable prospectus supplement information on the restrictions, elections, material federal income tax considerations, specific terms and other information with respect to that issue of debt securities and the foreign currency or currencies.

Unless otherwise specified in the prospectus supplement, the debt securities will be registered debt securities. Debt securities may be sold at a substantial discount below their stated principal amount, bearing no interest or interest at a rate which at the time of issuance is below market rates. The U.S. federal income tax considerations applicable to debt securities sold at a discount will be described in the applicable prospectus supplement.

Debt securities may be transferred or exchanged at the office of the security registrar or at the office of any transfer agent designated by us.

We will not impose a service charge for any transfer or exchange, but we may require holders to pay any tax or other governmental charges associated with any transfer or exchange.

In the event of any partial redemption of debt securities of any series, we will not be required to:

Initially, we will appoint the trustee as the security registrar. Any transfer agent, in addition to the security registrar initially designated by us, will be named in the prospectus supplement. We may designate additional transfer agents or change transfer agents or change the office of the transfer agent. However, we will be required to maintain a transfer agent in each place of payment for the debt securities of each series.

If any debt securities being offered are convertible into or exchangeable for our common stock or other securities, the relevant prospectus supplement will set forth the terms of conversion or exchange. Those terms will include whether conversion or exchange is mandatory, at the option of the holder or at our option, and the number of shares of common stock or other securities, or the method of determining the number of shares of common stock or other securities, to be received by the holder upon conversion or exchange. These provisions may allow or require the number of shares of our common stock or other securities to be received by the holders of such series of debt securities to be adjusted.

The debt securities of any series may be represented, in whole or in part, by one or more global securities. Each global security will:

No global security may be exchanged in whole or in part for debt securities registered in the name of any person other than the depositary or any nominee unless:

As long as the depositary, or its nominee, is the registered owner of a global security, the depositary or nominee will be considered the sole owner and holder of the debt securities represented by the global security for all purposes under the indenture. Except in the above limited circumstances, owners of beneficial interests in a global security will not be:

Payments on a global security will be made to the depositary or its nominee as the holder of the global security. Some jurisdictions have laws that require that certain purchasers of securities take physical delivery of such securities in definitive form. These laws may impair the ability to transfer beneficial interests in a global security.

Institutions that have accounts with the depositary or its nominee are referred to as "participants." Ownership of beneficial interests in a global security will be limited to participants and to persons that may hold beneficial interests through participants. The depositary will credit, on its book-entry registration and transfer system, the respective principal amounts of debt securities represented by the global security to the accounts of its participants.

Ownership of beneficial interests in a global security will be shown on and effected through records maintained by the depositary, with respect to participants' interests, or any participant, with respect to interests of persons held by participants on their behalf.

Payments, transfers and exchanges relating to beneficial interests in a global security will be subject to policies and procedures of the depositary. The depositary policies and procedures may change from time to time. Neither any trustee nor we will have any responsibility or liability for the depositary's or any participant's records with respect to beneficial interests in a global security.

Unless otherwise indicated in a prospectus supplement, the provisions described in this paragraph will apply to the debt securities. Payment of interest on a debt security on any interest payment date will be made to the person in whose name the debt security is registered at the close of business on the regular record date. Payment on debt securities of a particular series will be payable at the office of a paying agent or paying agents designated by us. However, at our option, we may pay interest by mailing a check to the record holder. The trustee will be designated as our initial paying agent.

We may also name any other paying agents in a prospectus supplement. We may designate additional paying agents, change paying agents or change the office of any paying agent. However, we will be required to maintain a paying agent in each place of payment for the debt securities of a particular series.

All moneys paid by us to a paying agent for payment on any debt security that remain unclaimed for a period ending the earlier of:

Unless otherwise indicated in a prospectus supplement with respect to a particular series of debt securities, the debt securities will not contain any provisions that may afford holders of the debt

securities protection in the event we have a change in control or in the event of a highly leveraged transaction, whether or not such transaction results in a change in control.

Unless otherwise indicated in a prospectus supplement with respect to a particular series of debt securities, the debt securities will not contain any financial or restrictive covenants.

Unless otherwise indicated in a prospectus supplement with respect to a particular series of debt securities, we may not consolidate with or merge into any other person, in a transaction in which we are not the surviving corporation, or convey, transfer or lease our properties and assets substantially as an entirety to, any entity, unless:

Unless we indicate otherwise in a prospectus supplement with respect to a particular series of debt securities, the following will be events of default for any series of debt securities under the indentures:

Additional or different events of default applicable to a series of debt securities may be described in a prospectus supplement. An event of default of one series of debt securities is not necessarily an event of default for any other series of debt securities.

The trustee may withhold notice to the holders of any default, except defaults in the payment of principal, premium, if any, interest, any sinking fund installment on, or with respect to any conversion right of, the debt securities of such series. However, the trustee must consider it to be in the interest of the holders of the debt securities of such series to withhold this notice.

Unless we indicate otherwise in a prospectus supplement, if an event of default, other than an event of default described in clause (5) above, shall occur and be continuing with respect to any series of debt securities, either the trustee or the holders of at least 25% in aggregate principal amount of the outstanding securities of that series may declare the principal amount and premium, if any, of the debt securities of that series, or if any debt securities of that series are original issue discount securities, such

other amount as may be specified in the applicable prospectus supplement, in each case together with accrued and unpaid interest, if any, thereon, to be due and payable immediately.

If an event of default described in clause (5) above shall occur, the principal amount and premium, if any, of all the debt securities of that series, or if any debt securities of that series are original issue discount securities, such other amount as may be specified in the applicable prospectus supplement, in each case together with accrued and unpaid interest, if any, thereon, will automatically become immediately due and payable without any declaration or other action on the part of the trustee or any holder. Any payment by us on the subordinated debt securities following any such acceleration will be subject to the subordination provisions described below under "Subordinated Debt Securities."

After acceleration, the holders of a majority in aggregate principal amount of the outstanding securities of that series may, under certain circumstances, rescind and annul such acceleration if all events of default, other than the non-payment of accelerated principal, or other specified amounts or interest, have been cured or waived.

Other than the duty to act with the required care during an event of default, the trustee will not be obligated to exercise any of its rights or powers at the request of the holders unless the holders shall have offered to the trustee reasonable indemnity. Generally, the holders of a majority in aggregate principal amount of the outstanding debt securities of any series will have the right to direct the time, method and place of conducting any proceeding for any remedy available to the trustee or exercising any trust or power conferred on the trustee.

A holder of debt securities of any series will not have any right to institute any proceeding under the indentures, or for the appointment of a receiver or a trustee, or for any other remedy under the indentures, unless:

Holders may, however, sue to enforce the payment of principal, premium or interest on any debt security on or after the due date or to enforce the right, if any, to convert any debt security (if the debt security is convertible) without following the procedures listed in clauses (1) through (3) above.

We will furnish the trustee an annual statement by our officers as to whether or not we are in default in the performance of the conditions and covenants under the indenture and, if so, specifying all known defaults.

Unless we indicate otherwise in a prospectus supplement, we and the applicable trustee may make modifications and amendments to an indenture with the consent of the holders of a majority in aggregate principal amount of the outstanding securities of each series affected by the modification or amendment.

We may also make modifications and amendments to the indentures for the benefit of holders without their consent, for certain purposes including, but not limited to:

However, neither we nor the trustee may make any modification or amendment without the consent of the holder of each outstanding security of that series affected by the modification or amendment if such modification or amendment would:

We may be discharged from our obligations on the debt securities, subject to limited exceptions, of any series that have matured or will mature or be redeemed within one year if we deposit enough money with the trustee to pay all of the principal, interest and any premium due to the stated maturity date or redemption date of the debt securities.

Each indenture contains a provision that permits us to elect either or both of the following:

To make either of the above elections, we must irrevocably deposit in trust with the trustee enough money to pay in full the principal, interest and premium on the debt securities. This amount may be made in cash and/or U.S. government obligations or, in the case of debt securities denominated in a currency other than U.S. dollars, cash in the currency in which such series of securities is denominated and/or foreign government obligations. As a condition to either of the above elections, for debt securities denominated in U.S. dollars we must deliver to the trustee an opinion of counsel that the holders of the debt securities will not recognize income, gain or loss for U.S. federal income tax purposes as a result of the action.

"Foreign government obligations" means, with respect to debt securities of any series that are denominated in a currency other than United States dollars:

Notices to holders will be given by mail to the addresses of the holders in the security register.

The indentures and the debt securities will be governed by, and construed under, the laws of the State of New York.

No incorporator, shareholder, employee, agent, officer, director or subsidiary of ours will have any liability for any obligations of ours, or because of the creation of any indebtedness under the debt securities, the indentures or supplemental indentures. The indentures provide that all such liability is expressly waived and released as a condition of, and as a consideration for, the execution of such indentures and the issuance of the debt securities.

The indentures limit the right of the trustee, should it become our creditor, to obtain payment of claims or secure its claims.

The trustee is permitted to engage in certain other transactions with us. However, if the trustee acquires any conflicting interest, and there is a default under the debt securities of any series for which it is trustee, the trustee must eliminate the conflict or resign.

The accompanying prospectus supplement will specify the trustee for the particular series of debt securities to be issued under the indentures.

The following provisions will be applicable with respect to each series of subordinated debt securities, unless otherwise stated in the prospectus supplement relating to that series of subordinated debt securities.

The indebtedness evidenced by the subordinated debt securities of any series is subordinated, to the extent provided in the subordinated indenture and the applicable prospectus supplement, to the prior payment in full, of all senior debt, including any senior debt securities, in cash or other payment satisfactory to the holders of senior debt.

Upon any distribution of our assets upon any dissolution, winding up, liquidation or reorganization, whether voluntary or involuntary, marshalling of assets, assignment for the benefit of creditors, or in bankruptcy, insolvency, receivership or other similar proceedings, payments on the subordinated debt securities will be subordinated in right of payment to the prior payment in full in cash or other payment satisfactory to holders of senior debt of all senior debt.

In the event of any acceleration of the subordinated debt securities of any series because of an event of default with respect to the subordinated debt securities of that series, holders of any senior debt would be entitled to payment in full in cash or other payment satisfactory to holders of senior debt of all senior debt before the holders of subordinated debt securities are entitled to receive any payment or distribution.

In addition, the subordinated debt securities will be structurally subordinated to all indebtedness and other liabilities of our subsidiaries, including trade payables and lease obligations. This occurs because our right to receive any assets of our subsidiaries upon their liquidation or reorganization, and your right to participate in those assets, will be effectively subordinated to the claims of that subsidiary's creditors, including trade creditors, except to the extent that we are recognized as a creditor of such subsidiary. If we are recognized as a creditor of that subsidiary, our claims would still be subordinate to any security interest in the assets of the subsidiary and any indebtedness of the subsidiary senior to us.

We are required to promptly notify holders of senior debt or their representatives under the subordinated indenture if payment of the subordinated debt securities is accelerated because of an event of default.

Under the subordinated indenture, we may also not make payment on the subordinated debt securities if:

No new payment blockage period may start on the basis of a nonpayment default unless at least 365 days have elapsed from the initial effectiveness of the immediately prior payment blockage notice.

No nonpayment default that existed or was continuing on the date of delivery of any payment blockage notice to the trustee shall be the basis for a subsequent payment blockage notice.

As a result of these subordination provisions, in the event of our bankruptcy, dissolution or reorganization, holders of senior debt may receive more, ratably, and holders of the subordinated debt securities may receive less, ratably, than our other creditors. The subordination provisions will not prevent the occurrence of any event of default under the subordinated indenture.

The subordination provisions will not apply to payments from money or government obligations held in trust by the trustee for the payment of principal, interest and premium, if any, on subordinated debt securities pursuant to the provisions described under "Satisfaction and Discharge; Defeasance," if the subordination provisions were not violated at the time the money or government obligations were deposited into trust.

If the trustee or any holder receives any payment that should not have been made to them in contravention of subordination provisions before all senior debt is paid in full in cash or other payment satisfactory to holders of senior debt, then such payment will be held in trust for the holders of senior debt.

Senior debt securities will constitute senior debt under the subordinated indenture.

Additional or different subordination provisions may be described in a prospectus supplement relating to a particular series of debt securities.

"Designated senior debt" means our obligations under any particular senior debt in which the instrument creating or evidencing the same or the assumption or guarantee thereof, or related agreements or documents to which we are a party, expressly provides that such indebtedness shall be designated senior debt for purposes of the subordinated indenture. The instrument, agreement or other document evidencing any designated senior debt may place limitations and conditions on the right of such senior debt to exercise the rights of designated senior debt.

"Indebtedness" means the following, whether absolute or contingent, secured or unsecured, due or to become due, outstanding on the date of the indenture for such series of securities or thereafter created, incurred or assumed:

"Senior debt" means the principal of, premium, if any, and interest, including all interest accruing subsequent to the commencement of any bankruptcy or similar proceeding, whether or not a claim for post-petition interest is allowable as a claim in any such proceeding, on, and all fees and other amounts payable in connection with, our indebtedness. Senior debt shall not include:

"Subsidiary" means an entity more than 50% of the outstanding voting stock of which is owned, directly or indirectly, by us or by one or more of our other subsidiaries or by a combination of us and our other subsidiaries. For purposes of this definition, "voting stock" means stock or other similar interests to us which ordinarily has or have voting power for the election of directors, or persons performing similar functions, whether at all times or only so long as no senior class of stock or other interests has or have such voting power by reason of any contingency.

We may elect to offer fractional interests in shares of preferred stock, in which case we will issue to the public receipts for depositary shares and each of these depositary shares will represent a fraction of a share of the applicable series of preferred stock, as set forth in the applicable prospectus supplement. The following summary of the terms of the depositary shares does not purport to be complete and is subject to, and is qualified in its entirety by reference to, the terms of the depositary shares and preferred stock, as well as the form of the deposit agreement, our Articles of Incorporation and the Articles of Amendment relating to the applicable series of preferred stock that are, or will be, filed with the SEC. Therefore, you should carefully consider the actual provisions of these documents.

Each owner of a depositary share will be entitled, in proportion to the applicable fractional interest in shares of preferred stock underlying that depositary share, to all rights and preferences of the preferred stock underlying that depositary share. These rights may include dividend, voting, redemption and liquidation rights.

The shares of preferred stock underlying the depositary shares will be deposited with a bank or trust company selected by us to act as depositary, under a deposit agreement between us, the depositary and the holders of the depositary receipts. The depositary will be the transfer agent, registrar and dividend disbursing agent for the depositary shares. The name and address of the principal executive office of the depositary will be included in the prospectus supplement relating to the issue.

The depositary shares will be evidenced by depositary receipts issued pursuant to the depositary agreement. Holders of depositary receipts agree to be bound by the deposit agreement, which requires holders to take certain actions such as filing proof of residence and paying certain charges.

The depositary will distribute cash dividends or other cash distributions, if any, received in respect of the series of preferred stock underlying the depositary shares to the record holders of depositary receipts in proportion to the number of depositary shares owned by those holders on the relevant record date. The relevant record date for depositary shares will be the same date as the record date for the preferred stock.

In the event of a distribution other than in cash, the depositary will distribute property received by it to the record holders of depositary receipts that are entitled to receive the distribution, unless the depositary determines that it is not feasible to make the distribution. If this occurs, the depositary, with our approval, may adopt another method for the distribution, including selling the property and distributing the net proceeds to the holders.

If a series of preferred stock underlying the depositary shares has a liquidation preference, in the event of our voluntary or involuntary liquidation, dissolution or winding up, holders of depositary shares will be entitled to receive the fraction of the liquidation preference accorded each share of the applicable series of preferred stock, as set forth in the applicable prospectus supplement.

If a series of preferred stock underlying the depositary shares is subject to redemption, the depositary shares will be redeemed from the proceeds received by the depositary resulting from the redemption, in whole or in part, of the preferred stock held by the depositary. Whenever we redeem

any preferred stock held by the depositary, the depositary will redeem, as of the same redemption date, the number of depositary shares representing the preferred stock so redeemed. The depositary will mail the notice of redemption to the record holders of the depositary receipts promptly upon receiving the notice from us and no fewer than 20 nor more than 60 days, unless otherwise provided in the applicable prospectus supplement, prior to the date fixed for redemption of the preferred stock.

After the date fixed for redemption, the depositary shares called for redemption will no longer be outstanding. When the depositary shares are no longer outstanding, all rights of the holders will end, except the right to receive money, securities or other property payable upon redemption.

Upon receipt of notice of any meeting at which the holders of preferred stock are entitled to vote, the depositary will mail the information contained in the notice of meeting to the record holders of the depositary receipts underlying the preferred stock. Each record holder of those depositary receipts on the record date will be entitled to instruct the depositary as to the exercise of the voting rights pertaining to the amount of preferred stock underlying that holder's depositary shares. The record date for the depositary will be the same date as the record date for the preferred stock. The depositary will try, as far as practicable, to vote the preferred stock underlying the depositary shares in accordance with these instructions. We will agree to take all action that may be deemed necessary by the depositary in order to enable the depositary to vote the preferred stock in accordance with these instructions. The depositary will not vote the preferred stock to the extent that it does not receive specific instructions from the holders of depositary receipts.

Owners of depositary shares will be entitled to receive upon surrender of depositary receipts at the principal office of the depositary and payment of any unpaid amount due to the depositary, the number of whole shares of preferred stock underlying their depositary shares.

Partial shares of preferred stock will not be issued. Holders of preferred stock will not be entitled to deposit the shares under the deposit agreement or to receive depositary receipts evidencing depositary shares for the preferred stock.

The form of depositary receipt evidencing the depositary shares and any provision of the deposit agreement may be amended by agreement between the depositary and us. However, any amendment that materially and adversely alters the rights of the holders of depositary shares, other than fee changes, will not be effective unless the amendment has been approved by at least a majority of the outstanding depositary shares. The deposit agreement may be terminated by the depositary or us only if:

We will pay all transfer and other taxes and governmental charges arising solely from the existence of the depositary arrangement. We will also pay charges of the depositary in connection with:

Holders of depositary receipts will pay transfer, income and other taxes and governmental charges and other specified charges as provided in the deposit agreement for their accounts. If these charges have not been paid, the depositary may:

The depositary will forward to the holders of depositary receipts all reports and communications we deliver to the depositary that we are required to furnish to the holders of the preferred stock. In addition, the depositary will make available for inspection by holders of depositary receipts at the principal office of the depositary, and at such other places as it may from time to time deem advisable, any reports and communications we deliver to the depositary as the holder of preferred stock.

Neither we nor the depositary will be liable if either we or the depositary are prevented or delayed by law or any circumstance beyond the control of either the depositary or us in performing our respective obligations under the deposit agreement. Our obligations and the depositary's obligations will be limited to the performance in good faith of our or the depositary's respective duties under the deposit agreement. Neither we nor the depositary will be obligated to prosecute or defend any legal proceeding in respect of any depositary shares or preferred stock unless satisfactory indemnity is furnished. The depositary and we may rely on:

The depositary may resign at any time by delivering a notice to us. We may remove the depositary at any time. Any such resignation or removal will take effect upon the appointment of a successor depositary and its acceptance of such appointment. The successor depositary must be appointed within 60 days after delivery of the notice for resignation or removal. The successor depositary must be a bank and trust company having its principal office in the United States of America and having a combined capital and surplus of at least $50,000,000.

We may issue warrants for the purchase of common stock, preferred stock, debt securities or any combination thereof. Warrants may be issued independently or together with common stock, preferred stock or debt securities and may be attached to or separate from any offered securities. Each series of warrants will be issued under a separate warrant agreement, the terms of which will be described in the applicable prospectus supplement. The following summary of the terms of the warrants does not purport to be complete and is subject to, and is qualified in its entirety by reference to, the terms of the warrants as well as the form of warrant agreement and the terms of the applicable underlying securities. Therefore, you should carefully consider the actual provisions of these documents.

The particular terms of any issue of warrants will be described in the prospectus supplement relating to the issue. Those terms may include:

Debt warrant certificates will be exchangeable for new debt warrant certificates of different denominations. Debt warrants may be exercised at the corporate trust office of the warrant agent or any other office indicated in the prospectus supplement. Prior to the exercise of their debt warrants, holders of debt warrants will not have any of the rights of holders of the debt securities purchasable upon exercise and will not be entitled to payment of principal or premium, if any, or interest on the debt securities purchasable upon exercise.

We may issue subscription rights to purchase shares of our common stock or preferred stock. These subscription rights may be issued independently or together with any other security offered hereby and may or may not be transferable by the shareholder receiving the subscription rights in such offering. In connection with any offering of subscription rights, we may enter into a standby arrangement with one or more underwriters or other purchasers pursuant to which the underwriters or other purchasers may be required to purchase any securities remaining unsubscribed for after such offering.

The prospectus supplement relating to any offering of subscription rights will include specific terms relating to the offering, including:

The preceding summary of the terms of the subscription rights does not purport to be complete and is subject to, and is qualified in its entirety by reference to, the terms of the subscription rights being offered, as well as any applicable subscription rights certificate and the terms of the securities to which the subscription rights relate. Therefore, you should carefully consider the actual provisions of the subscription right, and subscription agreement and the applicable securities.

We may issue units comprised of one or more of the other securities described in this prospectus or any prospectus supplement in any combination. Each unit will be issued so that the holder of the unit is also the holder, with the rights and obligations of a holder, of each security included in the unit. The units may be issued under unit agreements to be entered into between us and a unit agent, which may provide that the securities included in the unit may not be held or transferred separately, at any time or times before a specified date or upon the occurrence of a specified event or occurrence.

The preceding summary of the terms of the units does not purport to be complete and is subject to, and is qualified in its entirety by reference to, the terms of the units being offered, as well as any applicable unit agreement and the terms of the applicable classes of securities comprising the unit. Therefore, you should carefully consider the actual provisions of the unit, any unit agreement and the applicable securities.

We may sell the securities offered under this prospectus from time to time pursuant to underwritten public offerings, negotiated transactions, block trades or a combination of these methods or through underwriters, dealers or agents or directly to one or more purchasers. The securities may be distributed from time to time in one or more transactions at:

Each time that we sell securities pursuant to this prospectus, we will provide a prospectus supplement or supplements that will describe the method of distribution and set forth the terms and conditions of the offering of such securities, including the following information:

We may grant to the underwriters options to purchase additional securities at the public offering price, with additional underwriting commissions or discounts, as applicable, set forth in the prospectus supplement. The terms of any option to purchase additional securities will be set forth in the prospectus supplement for those securities.

We may engage in at-the-market offerings into an existing trading market in accordance with Rule 415(a)(4) under the Securities Act. Any at-the-market offering will be through an underwriter or underwriters acting as principal or agent for us.

We may issue to our existing security holders, through a dividend or similar distribution, subscription rights to purchase shares of our common stock or preferred stock, which may or may not be transferable. In any distribution of subscription rights to our existing security holders, if all of the underlying securities are not subscribed for, we may then sell the unsubscribed securities directly to third parties or may engage the services of one or more underwriters, dealers or agents, including standby underwriters, to sell the unsubscribed securities to third parties. The applicable prospectus supplement will describe the specific terms of any offering of our common stock or preferred stock through the issuance of subscription rights, including, if applicable, the material terms of any standby underwriting or purchase arrangement.

If underwriters are used in the sale, the underwriters will acquire the securities for their own account, including through underwriting, purchase, security lending or repurchase agreements with us. The underwriters may resell the securities from time to time in one or more transactions, including

negotiated transactions. Underwriters may sell the securities in order to facilitate transactions in any of our other securities (described in this prospectus or otherwise), including other public or private transactions and short sales. Underwriters may offer securities to the public either through underwriting syndicates represented by one or more managing underwriters or directly by one or more firms acting as underwriters. Unless otherwise indicated in the prospectus supplement, the obligations of the underwriters to purchase the securities will be subject to certain conditions, and the underwriters will be obligated to purchase all the offered securities if they purchase any of them. The underwriters may change from time to time any initial public offering price and any discounts or concessions allowed or reallowed or paid to dealers. The prospectus supplement will include the names of the underwriters and the terms of the transaction, including any underwriting discounts or commissions.

If dealers are used in the sale of securities offered through this prospectus, we will sell the securities to them as principals. They may then resell those securities to the public at varying prices determined by the dealers at the time of resale. The prospectus supplement will include the names of the dealers and the terms of the transaction, including any discounts or commissions received.

We may sell the securities offered through this prospectus directly. In this case, no underwriters, dealers or agents would be involved. Such securities may also be sold through agents designated from time to time. The prospectus supplement will name any agent involved in the offer or sale of the offered securities and will describe any commissions payable to the agent. Unless otherwise indicated in the prospectus supplement, any agent will agree to use its reasonable best efforts to solicit purchases for the period of its appointment.

We may sell the securities directly to institutional investors or others who may be deemed to be underwriters within the meaning of the Securities Act with respect to any sale of those securities. The terms of any such sales will be described in the prospectus supplement.

If the prospectus supplement indicates, we may authorize underwriters, dealers or agents to solicit offers from certain types of institutions to purchase securities at the public offering price under delayed delivery contracts. These contracts would provide for payment and delivery on a specified date in the future. The contracts would be subject only to those conditions described in the prospectus supplement. The applicable prospectus supplement will describe the commission payable for solicitation of those contracts.

Unless the applicable prospectus supplement states otherwise, all preferred stock, debt securities, depositary shares, warrants, subscription rights and units will be new issues of securities with no established trading market. Unless indicated in the applicable prospectus supplement, we do not expect to list the offered securities on a securities exchange, except for our common stock, which is listed on The Nasdaq Global Market. Any underwriters that we use in the sale of offered securities may make a market in such securities, but may discontinue such market making at any time without notice. Therefore, we cannot assure you that the securities will have a liquid trading market.

Any underwriter may also engage in stabilizing transactions, syndicate covering transactions and penalty bids in accordance with Rule 104 of Regulation M under the Exchange Act. Stabilizing transactions involve bids to purchase the underlying security in the open market for the purpose of preventing or retarding a decline in the price of the securities. Syndicate covering transactions involve purchases of the securities in the open market after the distribution has been completed in order to cover syndicate short positions. Penalty bids permit the underwriters to reclaim a selling concession

from a syndicate member when the securities originally sold by the syndicate member are purchased in a stabilizing or syndicate covering transaction to cover syndicate short positions.

Stabilizing transactions, syndicate covering transactions and penalty bids may have the effect of raising or maintaining the market price of our securities or preventing or retarding a decline in the market price of our securities. As a result, the price of the securities in the open market may be higher than it would otherwise be in the absence of these transactions. The underwriters may, if they commence these transactions, discontinue them at any time.

We, the underwriters or other agents may engage in derivative transactions involving the securities. These derivatives may consist of short sale transactions and other hedging activities. The underwriters or agents may acquire a long or short position in the securities, hold or resell securities acquired and purchase options or futures on the securities and other derivative instruments with returns linked to or related to changes in the price of the securities. In order to facilitate these derivative transactions, we may enter into security lending or repurchase agreements with the underwriters or agents. The underwriters or agents may effect the derivative transactions through sales of the securities to the public, including short sales, or by lending the securities in order to facilitate short sale transactions by others. The underwriters or agents may also use the securities purchased or borrowed from us or others (or, in the case of derivatives, securities received from us in settlement of those derivatives) to directly or indirectly settle sales of the securities or close out any related open borrowings of the securities.

We may also make sales through the Internet or through other electronic means. Since we may from time to time elect to offer securities directly to the public, with or without the involvement of agents, underwriters or dealers, utilizing the Internet or other forms of electronic bidding or ordering systems for the pricing and allocation of such securities, you will want to pay particular attention to the description of that system, which we will provide in a prospectus supplement.

Such electronic system may allow bidders to directly participate, through electronic access to an auction site, by submitting conditional offers to buy that are subject to acceptance by us, and which may directly affect the price or other terms and conditions at which such securities are sold. These bidding or ordering systems may present to each bidder, on a so-called "real-time" basis, relevant information to assist in making a bid, such as the clearing spread at which the offering would be sold, based on the bids submitted, and whether a bidder's individual bids would be accepted, prorated or rejected. For example, in the case of debt security, the clearing spread could be indicated as a number of "basis points" above an index treasury note.

Upon completion of such an electronic auction process, securities will be allocated based on prices bid, terms of bid or other factors. The final offering price at which securities would be sold and the allocation of securities among bidders would be based in whole or in part on the results of the Internet or other electronic bidding process or auction.

The specific terms of any lock-up provisions in respect of any given offering will be described in the applicable prospectus supplement.

Underwriters, dealers and agents may be entitled, under agreements entered into with us, to indemnification by us against certain liabilities, including liabilities under the Securities Act. Our underwriters, dealers and agents, or their affiliates, may be customers of, engage in transactions with or perform services for us, in the ordinary course of business.

In compliance with guidelines of the Financial Industry Regulatory Authority, Inc. ("FINRA"), and unless otherwise disclosed in the applicable prospectus supplement, we do not intend for the maximum consideration or discount to be received by any FINRA member or independent broker dealer to exceed 8.0% of the aggregate amount of the securities offered pursuant to this prospectus and any applicable prospectus supplement.

Underwriters, dealers and agents may engage in transactions with us, or perform services for us, in the ordinary course of business for which they receive customary compensation.

The legality of any offered securities will be passed upon for us by Covington & Burling LLP, Washington, D.C., and Keller Rohrback L.L.P., Seattle, Washington. Counsel for any underwriters, dealers or agents will be named in the applicable prospectus supplement.

Title of Each Class of Securities to be Registered	Amount to be Registered⁽¹⁾	Proposed Maximum Offering Price per Security	Proposed Maximum Aggregate Offering Price	Amount of Registration Fee⁽²⁾




Common Stock, par value $0.01 per share	4,389,311	$13.10	$57,499,974.10	$7,463.50

	Per Share	Total




Public offering price	$13.10	$49,999,988

Underwriting fees and discount⁽¹⁾	$0.68	$2,595,419

Proceeds before expenses, to us	$12.42	$47,404,569

		Page

About this Prospectus Supplement			ii
Forward-Looking Statements			iii
Prospectus Supplement Summary			S-1
The Offering			S-6
Risk Factors			S-7
Use of Proceeds			S-9
Dilution			S-10
Dividend Policy			S-12
Material U.S. Federal Tax Consequences to Non-U.S. Holders			S-13
Underwriting			S-18
Legal Matters			S-26
Experts			S-26
Where You Can Find More Information			S-26
Incorporation of Documents By Reference			S-27


Assumed public offering price per share				$	13.10
Net tangible book deficit per share as of September 30, 2019	$	(2.83	)
Increase in net tangible book value per share attributable to this offering	$	1.09

As adjusted net tangible book value per share on September 30, 2019 after this offering	$	(1.74	)
Dilution per share to new investors purchasing our common stock in this offering				$	14.84

			Total

	Per Share		No Exercise		Full Exercise
Public offering price	$	13.10	$	49,999,988	$	57,499,974
Underwriting fees and discount	$	0.68	$	2,595,419	$	2,984,731
Proceeds to us, before expenses	$	12.42	$	47,404,569	$	54,515,243

		Page

About this Prospectus			ii
Forward-Looking Statements			iii
About Omeros			1
Where You Can Find More Information			2
Incorporation by Reference			3
Risk Factors			3
Use of Proceeds			4
Description of Common Stock			5
Description of Preferred Stock			8
Description of Debt Securities			11
Description of Depository Shares			22
Description of Warrants			25
Description of Subscription Rights			26
Description of Units			27
Plan of Distribution			28
Legal Matters			32
Experts			32