Conversation so far:
What did John O. Brennan do as an advisor to Barack Obama?
John O. Brennan served as chief counterterrorism advisor to U.S. President Barack Obama, with the title Deputy National Security Advisor for Homeland Security and Counterterrorism, and Assistant to the President.

Question: When did he begin in that position?
Self-contained rewrite: When did John O. Brennan begin as Deputy National Security Advisor for Homeland Security and Counterterrorism?