Exhibit 5.1

	ROPES & GRAY LLP
	ONE INTERNATIONAL PLACE            BOSTON, MA 02110-2624            617-951-7000            F 617-951-7050
	BOSTON        NEW YORK        PALO ALTO        SAN FRANCISCO        WASHINGTON, DC        www.ropesgray.com

October 27, 2006

Oscient Pharmaceuticals Corporation

1000 Winter Street, Suite 2200

Waltham, Massachusetts 02451

Ladies and Gentlemen:

This opinion is furnished to you in connection with a registration statement on Form S-8 and all exhibits thereto (the “Registration Statement”), filed with the Securities and Exchange Commission under the Securities Act of 1933, as amended, for the registration of 5,750,000 shares of Common Stock, $.10 par value (the “Shares”), of Oscient Pharmaceuticals Corporation, a Massachusetts corporation (the “Company”). 750,000 of the Shares being registered are issuable pursuant to the Company’s Amended and Restated Employee Stock Purchase Plan (the “Employee Stock Purchase Plan”), and 5,000,000 of the Shares registered are issuable pursuant to the Company’s Amended and Restated 2001 Incentive Plan (the “2001 Incentive Plan,” and together with the Employee Stock Purchase Plan, the “Plans”), each of which are attached as an exhibit to the Registration Statement.

We are familiar with the actions taken by the Company in connection with the adoption of the Plans. For purposes of our opinion, we have examined and relied upon such documents, records, certificates and other instruments as we have deemed necessary. The opinions expressed below are limited to the laws of The Commonwealth of Massachusetts.

Based on the foregoing, we are of the opinion that the Shares have been duly authorized and, when the Shares have been issued and sold in accordance with the terms of the Plans, the Shares will have been validly issued, fully paid and nonassessable.

We hereby consent to the filing of this opinion as an exhibit to the Registration Statement. Our consent shall not be deemed an admission that we are experts whose consent is required under Section 7 of the Securities Act of 1933.

ROPES & GRAY LLP

It is understood that this opinion is to be used only in connection with the offer and sale of Shares while the Registration Statement is in effect.

Very truly yours,

/s/ ROPES & GRAY LLP