Following the 2003 invasion of Iraq, she volunteered for the Office for Reconstruction and Humanitarian Assistance under Jay Garner. Defense Secretary Donald Rumsfeld told Jay Garner that he could not keep her (or Tom Warrick) on in Iraq, though Rumsfeld later relented. She was an assistant to Paul Bremer in the Coalition Provisional Authority. She was Senior Director for Iraq 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 where she frequently communicated via telephone with Fort Leavenworth's General David Petraeus on a new military strategy for Iraq.