EXHIBIT 5.1

Yvan-Claude Pierre

+1 212 479 6721

ypierre@cooley.com

February 15, 2017

Oncobiologics, Inc.

7 Clarke Drive

Cranbury, New Jersey 08512

Ladies and Gentlemen:

You have requested our opinion, as counsel to Oncobiologics, Inc., a Delaware corporation (the “Company”), in connection with the filing by the Company of a Registration Statement on Form S-1 (the “Registration Statement”) with the Securities and Exchange Commission, including a related prospectus filed with the Registration Statement (the “Prospectus”), covering the registration for the offer and resale of up to 2,024,000 shares (the “Warrant Shares”) of common stock, par value $0.01 per share (the “Common Stock”), issuable upon exercise of the warrants (the “Warrants”) held by the selling stockholders identified in the Prospectus. The Warrants were issued pursuant to a note and warrant purchase agreement between the Company and the purchasers party thereto, dated as of December 22, 2016 (the “NWPA”).

In connection with this opinion, we have examined and relied upon (a) the Registration Statement and the Prospectus, (b) the Company’s Amended and Restated Certificate of Incorporation and Amended and Restated Bylaws, as amended, each as currently in effect, (c) the NWPA, including the forms of the Warrants attached thereto, and (d) originals or copies certified to our satisfaction of such records, documents, certificates, memoranda and other instruments as in our judgment are necessary or appropriate to enable us to render the opinion expressed below. We have undertaken no independent verification with respect to such matters. We have assumed the genuineness and authenticity of all documents submitted to us as originals, the conformity to originals of all documents submitted to us as copies thereof, the accuracy, completeness and authenticity of certificates of public officials and the due execution and delivery of all documents by persons other than the Company where due execution and delivery are a prerequisite to the effectiveness thereof. As to certain factual matters, we have relied upon a certificate of an officer of the Company and have not sought independently to verify such matters.

With regard to our opinion as to the Warrant Shares, (i) we have assumed that the exercise price of the Warrants at the time of exercise is equal to or greater than the par value of the Common Stock, and (ii) we express no opinion to the extent that, notwithstanding its current reservation of shares of Common Stock, future issuances of securities, including the Warrant Shares, of the Company and/or antidilution adjustments to outstanding securities, including the Warrants, of the Company cause the Warrants to be exercisable for more shares of Common Stock than the number that then remain authorized but unissued.

Our opinion is expressed only with respect to the General Corporation Law of the State of Delaware. We express no opinion as to whether the laws of any particular jurisdiction are applicable to the subject matter hereof.  We are not rendering any opinion as to compliance with any federal or state antifraud law, rule or regulation relating to securities, or to the sale or issuance thereof.

On the basis of the foregoing, and in reliance thereon, we are of the opinion that the Warrant Shares, when issued and sold by the Company in accordance with the Warrants, will be validly issued, fully paid and nonassessable.

Cooley LLP   1114 Avenue of the Americas   New York, NY  10036

t: (212) 479-6000  f: (212) 479-6275  cooley.com

February 15, 2017

Page Two

We consent to the reference to our firm under the caption “Legal Matters” in the Prospectus included in the Registration Statement and to the filing of this opinion as an exhibit to the Registration Statement.

Sincerely,

Cooley LLP
By:	/s/ Yvan-Claude Pierre
	Yvan-Claude Pierre

Cooley LLP   1114 Avenue of the Americas   New York, NY  10036

t: (212) 479-6000  f: (212) 479-6275  cooley.com