[2] She is a member of the board of directors of the Council on Foreign Relations,[3] and Raytheon,[4][5] and the North American chair of the Trilateral Commission.

Starting July 2004, she was Senior Director for Iraq and Afghanistan at the United States National Security Council.

O'Sullivan's last position at the White House was as the Special Assistant to the President and Deputy National Security Advisor for Iraq and Afghanistan from October 2005 to September 2007 where she frequently communicated via telephone with Fort Leavenworth's General David Petraeus on a new military strategy for Iraq.

She is remembered for driving herself around Baghdad to meet with Iraqis, and endured some harrowing experiences while in Iraq, including escaping from a terrorist attack by scaling a building ledge ten stories up.

[16] In 2013, O'Sullivan was a signatory to an amicus curiae brief submitted to the Supreme Court in support of same-sex marriage during the Hollingsworth v. Perry case.

[23] In April 2021, the Washington Post issued a correction to an op-ed by O'Sullivan after the original version failed to disclose her board membership at Raytheon Corp, one of the largest weapons manufacturers in the world.