Exhibit 3.1

THIRD AMENDED AND RESTATED

CERTIFICATE OF INCORPORATION

OF

US ONCOLOGY HOLDINGS, INC.,

a Delaware corporation

US ONCOLOGY HOLDINGS, INC., a corporation organized and existing under the General Corporation Law of the State of Delaware (the “ Corporation ”)

DOES HEREBY CERTIFY:

1. That the name of the Corporation is US Oncology Holdings, Inc. The name under which the Corporation was originally incorporated is Oiler Holding Company. The original Certificate of Incorporation of the Corporation was filed with the Secretary of State of Delaware on March 17, 2004. An Amended and Restated Certificate of Incorporation was filed with the Secretary of State of Delaware on August 19, 2004. A Second Amended and Restated Certificate of Incorporation was filed with the Secretary of State of Delaware on December 21, 2006.

2. That pursuant to Sections 228, 242 and 245 of the General Corporation Law of the State of Delaware, this Third Amended and Restated Certificate of Incorporation amends and restates the provisions of the Second Amended and Restated Certificate of Incorporation of the Corporation in its entirety.

3. That the Board of Directors of the Corporation, at a meeting on September 17, 2009 adopted resolutions setting forth a proposed amendment and restatement of the Second Amended and Restated Certificate of Incorporation of the Corporation in the form hereof, declaring said amendment and restatement to be advisable and in the best interests of the Corporation and its stockholders and submitting the proposed amendment and restatement to the stockholders of the Corporation for consideration thereof.

4. That, as required under the Second Amended and Restated Certificate of Incorporation, this Third Amended and Restated Certificate of Incorporation was: (i) approved by written consent, dated September 30, 2009, by the holders of more than 66 and 2/3% of the outstanding shares of the Series A Participating Preferred Stock of the Corporation (the “ Series A Preferred Stock ”); (ii) approved by written consent, dated September 30, 2009, by the holders of a majority of the Series A-1 Participating Preferred Stock of the Corporation (the “ Series A-1 Preferred Stock ”); and (iii) approved by written consent, dated September 30, 2009, by the holders of a majority of the Common Stock, par value $0.001 per share, of the Corporation (the “ Common Stock ”).

5. Immediately after giving effect to the filing with the Secretary of State of the State of Delaware of this Third Amended and Restated Certificate of Incorporation of the Corporation (such time, the “ Conversion Effective Time ”), the Series A Preferred Stock and the Series A-1 Preferred Stock (together, the “ Participating Preferred Stock ”) shall be converted so that each share of Participating Preferred Stock that is issued and outstanding immediately prior to the Conversion Effective Time (the “ Existing Participating Preferred Stock ”) shall be automatically converted, without further action on the part of the Corporation or any holder of Participating Preferred Stock, into some number of fully paid and non-assessable whole shares of Common Stock equal to the sum of (with the capitalized terms used herein but not otherwise defined having the meaning set forth in the Second Amended and Restated Cer