Exhibit 5.1

November 18, 2016

Board of Directors

TESARO, Inc.

1000 Winter Street

Suite 3300

Waltham, MA 02451

Ladies and Gentlemen:

We are acting as counsel to TESARO, Inc., a Delaware corporation (the “Company”), in connection with its registration statement on Form S-3, as amended (File No. 333-212324) (the “Registration Statement”), filed with the Securities and Exchange Commission under the Securities Act of 1933, as amended (the “Act”) relating to the public offering of certain securities of the Company that may be offered and sold from time to time and on a delayed or a continuous basis as set forth in the Prospectus dated June 30, 2016 (the “Prospectus”) and the proposed public offering of up to 1,750,000 shares of the common stock, par value $0.0001 per share (the “Common Stock”) of the Company (the “Shares”), all of which shares are to be sold by the Company pursuant to the Underwriting Agreement, dated as of November 15, 2016, among the Company and the underwriters named therein (the “Underwriting Agreement”), as described in the supplement to the Prospectus dated November 15, 2016 (the “Prospectus Supplement”).  This opinion letter is furnished to you at your request to enable you to fulfill the requirements of Item 601(b)(5) of Regulation S-K, 17 C.F.R. § 229.601(b)(5), in connection with the Registration Statement.

For purposes of this opinion letter, we have examined copies of such agreements, instruments and documents as we have deemed an appropriate basis on which to render the opinions hereinafter expressed.  In our examination of the aforesaid documents, we have assumed the genuineness of all signatures, the legal capacity of all natural persons, the accuracy and completeness of all documents submitted to us, the authenticity of all original documents, and the conformity to authentic original documents of all documents submitted to us as copies (including telecopies).  As to all matters of fact, we have relied on the representations and statements of fact made in the documents so reviewed, and we have not independently established the facts so relied on.  This opinion letter is given, and all statements herein are made, in the context of the foregoing.

This opinion letter is based as to matters of law solely on the Delaware General Corporation Law, as amended. We express no opinion herein as to any other statutes, rules or regulations.

Based upon, subject to and limited by the foregoing, we are of the opinion that following (i) issuance of the Shares pursuant to the terms of the Underwriting Agreement, and (ii) receipt by the Company of the consideration for the Shares specified in the resolutions of the Board of Directors and the

Hogan Lovells US LLP is a limited liability partnership registered in the District of Columbia.  “Hogan Lovells” is an international legal practice that includes Hogan Lovells US LLP and Hogan Lovells International LLP, with offices in:  Alicante   Amsterdam   Baltimore   Beijing   Brussels   Caracas   Colorado Springs   Denver   Dubai   Dusseldorf   Frankfurt   Hamburg   Hanoi   Ho Chi Minh City   Hong Kong   Houston   Johannesburg   London   Los Angeles   Luxembourg   Madrid   Mexico City   Miami   Milan   Minneapolis   Monterrey   Moscow   Munich   New York   Northern Virginia   Paris   Perth   Philadelphia   Rio de Janeiro   Rome   San Francisco   São Paulo   Shanghai   Silicon Valley   Singapore   Sydney   Tokyo   Ulaanbaatar   Warsaw   Washington DC   Associated offices: Budapest   Jeddah   Riyadh   Zagreb.  For more information see www.hoganlovells.com

Pricing Committee of the Board of Directors, the Shares will be validly issued, fully paid, and nonassessable.

This opinion letter has been prepared for use in connection with the Registration Statement.  We assume no obligation to advise you of any changes in the foregoing subsequent to the date hereof.

We hereby consent to the filing of this opinion letter as Exhibit 5.1 to the Company’s Current Report on Form 8-K to be filed with the Securities and Exchange Commission on the date hereof and thereby incorporated by reference as an exhibit to the Registration Statement and to the references to this firm under the caption “Legal Matters” in the Prospectus and the Prospectus Supplement, respectively.  In giving this consent, we do not thereby admit that we are an “expert” within the meaning of the Act.

Very truly yours,

/s/ HOGAN LOVELLS US LLP

HOGAN LOVELLS US LLP

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