SC TO-I/A

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE TO

(AMENDMENT NO. 1)

(Rule 14d-100)

TENDER OFFER STATEMENT UNDER SECTION 14(d)(1) OR SECTION 13(e)(1)

OF THE SECURITIES EXCHANGE ACT OF 1934

Oscient Pharmaceuticals Corporation

(Name of Subject Company (Issuer) and Filing Person (Offeror))

3 ¹/2% Senior Convertible Notes due 2011 and

5% Convertible Promissory Notes due 2009

(Title of Class of Securities)

68812RAA3, 68812RAB1 and

n/a

(CUSIP Number of Class of Securities)

Philippe M. Maitre

Oscient Pharmaceuticals Corporation

1000 Winter Street, Suite 2200

Waltham, Massachusetts 02451

(781) 398-2300

(Name, Address and Telephone Number of Person Authorized to Receive

Notices and Communications on Behalf of the Person(s) Filing Statement)

Copies to:

Joseph D. Vittiglio, Esq. Oscient Pharmaceuticals Corporation 1000 Winter Street, Suite 2200 Waltham, MA 02451 (781) 398-2300 Patrick O’Brien, Esq. Ropes & Gray LLP One International Place Boston, MA 02110 (617) 951-7000 Danielle Carbone, Esq. Shearman & Sterling LLP 599 Lexington Avenue New York, NY 10022 (212) 848-4000 Abigail Arms, Esq. Shearman & Sterling LLP 801 Pennsylvania Avenue, N.W. Washington, D.C. 20004 (202) 508-8000

CALCULATION OF FILING FEE

Transaction Valuation*	Amount Of Filing Fee**
$129,682,298	$3,981.25

* Pursuant to Rule 457(f) under the Securities Act of 1933, as amended, this amount is the market value as of March 9, 2007 of the aggregate principal amount outstanding of 3 ¹/2% Senior Convertible Notes due 2011 and the book value of the aggregate principal amount outstanding of 5% Convertible Promissory Notes due 2009 that may be received by the Registrant from tendering holders in the exchange offers and also includes the amount of new notes that the Registrant may issue to tendering holders of its 5% Convertible Promissory Notes due 2009 in exchange for accrued and unpaid interest on such tendered notes.

** Registration fee previously paid in connection with the Issuer’s Registration Statement on Form S-4 filed March 15, 2007 (File No. 333-141308), which fee was offset against a fee previously paid in connection with the Issuer’s Registration Statement on Form S-3 filed September 30, 2005 (File No. 333-128735).

x Check the box if any part of the fee is offset as provided by Rule 0-11(a)(2) and identify the filing with which the offsetting fee was previously paid. Identify the previous filing by registration statement number or the Form or Schedule and the date of its filing.

Amount Previously Paid:	$3,981.25
Form or Registration No.:	S-4 (333-141308)
Filing Party:	Oscient Pharmaceuticals Corporation
Date Filed:	March 15, 2007

¨ Check the box if the filing relates solely to preliminary communications made before the commencement of a tender offer.

Check the appropriate boxes below to designate any transactions to which the statement relates:

¨ third-party tender offer subject to Rule 14d-1.

x issuer tender offer subject to Rule 13e-4.

¨ going-private transaction subject to Rule 13e-3.

¨ amendment to Schedule 13D under Rule 13d-2.

Check the following box if the filing is a final amendment reporting the results of the tender offer: ¨

This Amendment No. 1 amends and supplements the Issuer Tender Offer Statement on Schedule TO (the “Schedule TO”) filed by Oscient Pharmaceuticals Corporation, a Massachusetts corporation (the “Company”), in connection with others (the “Exchange Offers”) to exchange its 3.50% Convertible Senior Notes due 2011 (the “New Notes”) for (i) its outstanding 3 ¹/2% Senior Convertible Notes due 2011 (the “Existing 2011 Notes”) and (ii) its outstanding 5% Convertible Promissory Notes due 2009 (the “Existing 2009 Notes” and, together with the Existing 2011 Notes, the “Existing Notes”), upon the terms and subject to the conditions set forth in the prospectus (as amended and supplemented from time to time, the “Prospectus”) and the related letters of transmittal (the “Letters of Transmittal”), which are parts of the Company’s Registration Statement on Form S-4 (File No. 333-141308), originally filed on March 15, 2007 (the “Registration Statement”) and are incorporated herein by reference.

ITEM 12. EXHIBITS.

See Exhibit Index immediately following the signature page to this Amendment No. 1

SIGNATURES

After due inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

OSCIENT PHARMACEUTICALS CORPORATION

By:  /s/  STEVEN M. RAUSCHER                            

Name: Steven M. Rauscher

Title: President and Chief Executive Officer

Dated: April 18, 2007

EXHIBIT INDEX

Exhibit No.	Description
(a)(1)(A)	Prospectus, dated March 29, 2007, incorporated by reference to the Company’s Registration Statement on Form S-4 (File No. 333-141308)
(a)(1)(B)	Forms of Letters of Transmittal (1)
(a)(1)(C)	Form of Notice of Guarantee of Delivery (1)
(a)(1)(D)	Form of Letter to Brokers, Dealers, Commercial Banks, Trust Companies and Others (1)
(a)(1)(E)	Form of Letters to Client (1)
(a)(1)(F)	Press Release issued March 29, 2007 (previously filed)
(a)(1)(G)	Prospectus, dated April 18, 2007, incorporated by reference to the Company’s Registration Statement on Form S-4 (File No. 333-141308)
(a)(1)(H)	Press release issued April 18, 2007
(a)(4)	Incorporated herein by reference to Exhibit (a)(1)(A) and (a)(1)(G)
(b)	Not applicable
(d)(1)	Form of Purchase Warrant issued to Smithfield Fiduciary LLC and the Tail Wind Fund Ltd. (2)
(d)(2)	Form of Common Stock Purchase Warrant dated as of September 29, 2003 (3)
(d)(3)	Registration Rights Agreement dated September 29, 2003 (3)
(d)(4)	Registration Rights Agreement dated November 17, 2003, by and between Genome Therapeutics Corp. and certain creditors of GeneSoft Pharmaceuticals, Inc. (4)
(d)(5)	Form of Indenture dated as of May 10, 2004 (5)
(d)(6)	Pledge Agreement dated as of May 10, 2004 (5)
(d)(7)	Registration Rights Agreement dated May 10, 2004 (5)
(d)(8)	Form of Indenture dated as of May 10, 2004 (5)
(d)(9)	Pledge Agreement dated May 10, 2004 (5)
(d)(10)	Registration Rights Agreement dated May 10, 2004 (5)
(d)(11)	Form of Common Stock Purchase Warrant dated April 5, 2006 (6)
(d)(12)	Form of Common Stock Purchase Warrant dated August 18, 2006 (7)
(d)(13)	Registration Rights Agreement dated August 18, 2006 (7)
(d)(14)	Incentive Stock Option Plan and Form of Stock Option Certificate (8)
(d)(15)	Genome Therapeutics Corp. (f/k/a Collaborative Research) Incentive Savings Plan (8)
(d)(16)	Amendment dated November 4, 1986 to the Genome Therapeutics Corp. (f/k/a Collaborative Research) Incentive Savings Plan dated March 1, 1985 (10)
(d)(17)	1991 Stock Option Plan and Form of Stock Option Certificate (11)

Exhibit No.	Description
(d)(18)	Lease dated June 23, 2004 relating to certain property in Waltham, Massachusetts (7)
(d)(19)	1993 Stock Option Plan and Form of Stock Option Certificate (12)
(d)(20)	1997 Directors’ Deferred Stock Plan (13)
(d)(21)	1997 Stock Option Plan (13)
(d)(22)	Amended and Restated 2001 Incentive Plan (14)
(d)(23)	Stock Option Agreements with Steven M. Rauscher (15)
(d)(24)	Amendment, Redemption and Exchange Agreement between the Company and The Tail Wind Fund, dated June 4, 2003 (2)
(d)(25)	Note Amendment and Exchange Agreement dated November 17, 2003, by and between Genome Therapeutics Corp. and certain creditors of GeneSoft Pharmaceuticals, Inc. (16)
(d)(26)	Genome Therapeutics Corp. Employee Stock Purchase Plan as amended through April 13, 2004 (17)
(d)(27)	Genome Therapeutics Corp. 2001 Incentive Plan as amended through April 13, 2004 (17)
(d)(28)	Form of Incentive Stock Option (18)
(d)(29)	Form of Nonstatutory Stock Option (18)
(d)(30)	Form of Restricted Stock Award (18)
(d)(31)	Amended and Restated Employee Stock Purchase Plan (as amended through June 8, 2006) (14)
(d)(32)	Common Stock and Warrant Purchase Agreement dated July 21, 2006 by and between Oscient Pharmaceuticals Corporation and Paul Royalty Fund Holdings II (18)
(d)(33)	Note Purchase Agreement dated July 21, 2006 by and between Guardian Acquisition Corporation and Paul Royalty Fund Holdings II * (18)
(g)	Incorporated herein by reference to Exhibits (a)(1)(A), (a)(1)(B), (a)(1)(C), (a)(1)(D), (a)(1)(E) and (a)(1)(G)

(1) Filed as an exhibit to the Company’s Registration Statement on Form S-4 (File No. 333-141308) and incorporated herein by reference.

(2) Filed as an exhibit to the Company’s Current Report on Form 8-K on June 5, 2003 and incorporated herein by reference.

(3) Filed as an exhibit to the Company’s Current Report on Form 8-K on October 1, 2003 and incorporated herein by reference.

(4) Filed as an exhibit to the Company’s Registration Statement on Form S-4 (No. 333-111171) on September 15, 2003 and incorporated herein by reference.

(5) Filed as an exhibit to the Company’s Registration Statement on Form S-3 (333-118026) on August 9, 2004 and incorporated herein by reference.

(6) Filed as an exhibit to the Company’s Current Report on Form 8-K on April 12, 2006 and incorporated herein by reference.

(7) Filed as an exhibit to the Company’s Annual Report on Form 10-K for the year ended December 31, 2006 and incorporated herein by reference.

(8) Filed as an exhibit to the Company’s Registration Statement on Form S-1 (No. 2-75230) dated December 8, 1981 and incorporated herein by reference.

(9) Filed as an exhibit to the Company’s Annual Report on Form 10-K for the fiscal year ended August 31, 1985 and incorporated herein by reference.

(10) Filed as exhibits to the Company’s Annual Report on Form 10-K for the fiscal year ended August 31, 1986 and incorporated herein by reference.

(11) Filed as an exhibit to the Company’s Annual Report on Form 10-K for the fiscal year ended August 31, 1992 and incorporated herein by reference.

(12) Filed as exhibits to the Company’s Annual Report on Form 10-K for the fiscal year ended August 31, 1994 and incorporated herein by reference.

(13) Filed as exhibits to the Company’s Registration Statement on Forms S-8 (333-49069) dated April 1, 1998 and incorporated herein by reference.

(14) Filed as an exhibit to the Company’s Registration Statement on Form S-8 (333-138309) on October 30, 2006 and incorporated herein by reference.

(15) Filed as an exhibit to the Company’s Registration Statement on Form S-8 (333-58274) on April 4, 2001and incorporated herein by reference.

(16) Filed as an exhibit to the Company’s Quarterly Report on Form 10-Q for the quarter ended March 27, 2004.

(17) Filed as an exhibit to the Company’s Registration Statement on Form S-8 (333-116707) on June 21, 2004 and incorporated herein by reference.

(18) Filed as an exhibit to the Company’s Quarterly Report on Form 10-Q for the quarter ended September 30, 2006 and incorporated herein by reference.

* Confidential treatment requested with respect to portions of this Exhibit

EX-99.(A)(1)(H)

Exhibit (a)(1)(H)

Investor Relations:

Christopher Taylor

781-398-2466

Media Relations:

Sarah Emond

781-398-2544

For Immediate Release

Oscient Pharmaceuticals Announces Increase in Offering Size of New 3.50% Convertible Senior Notes Due 2011

Waltham, Mass., April 18, 2007 – Oscient Pharmaceuticals Corporation (Nasdaq: OSCI) today announced that it has filed, with the Securities and Exchange Commission, an amendment to its registration statements increasing the aggregate principal amount of its new money offering of 3.50% Convertible Senior Notes due 2011 from $30,000,000 to $50,000,000.

The Company currently expects that the new 3.50% Convertible Senior Notes due 2011 to be sold in the new money offering will be issued at between 72.5% and 77.5% of the principal amount (plus accrued and unpaid interest on the 2011 notes). The new money offering is being made on a best efforts basis and there is no minimum amount of new notes that the Company is required to sell.

Piper Jaffray & Co. is serving as the dealer manager for the exchange offers and placement agent for the new money offering. The exchange offers are scheduled to expire at 11:59 p.m., New York City Time, on April 25, 2007 unless extended.

A tender offer statement, registration statements (and the prospectus included therein), related letters of transmittal and other offering documents relating to these securities have been filed with the Securities and Exchange Commission but the registration statements have not yet become effective. These documents contain important information that should be read carefully before any decision is made with respect to the exchange offers or the new money offering. These securities may not be exchanged or sold, nor may offers to exchange or offers to buy them be accepted prior to the time the registration statements become effective. This news release shall not constitute an offer to exchange or sell or the solicitation of an offer to buy nor shall there be any offer, exchange or sale of these securities in any state or other jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such state.

With respect to the exchange offers, copies of the prospectus and related offer documents can be obtained by holders of the Company’s existing notes free of charge from the information agent, Georgeson Inc., 17 State Street, 10th Floor, New York, NY 10004, (888) 549-6633. With respect to the offer of new notes for cash, a copy of the prospectus may be obtained from Piper Jaffray & Co. at 345 California Street, Suite 2300, San Francisco, CA 94104, (415) 984-5141. The

-more-

Registration Amendment

April 18, 2007

Page 2 of 2

prospectus related to the exchange offers and new money offering may also be obtained free of charge at the Securities and Exchange Commission’s website (www.sec.gov).

About Oscient Pharmaceuticals

Oscient Pharmaceuticals Corporation is a commercial-stage biopharmaceutical company marketing two FDA-approved products through its national primary care sales force. ANTARA^® (fenofibrate) capsules is FDA-approved for the adjunct treatment of hypercholesterolemia (high blood cholesterol) and hypertriglyceridemia (high triglycerides) in combination with diet. FACTIVE^® (gemifloxacin mesylate) tablets is an antibiotic FDA-approved for the treatment of acute bacterial exacerbations of chronic bronchitis and community-acquired pneumonia of mild to moderate severity. Oscient also has a novel, late-stage antibiotic candidate, Ramoplanin, under investigation for the treatment of Clostridium difficile-associated disease (CDAD).

For important information regarding the safety and use of ANTARA and FACTIVE, please see the full prescribing information available at www.antararx.com and www.factive.com.