Former detective with CIA connections. Profiled in S1 "Voices". Identified Nikita in 1997 during a rape attempt witnessed by an elderly woman. When he continues his pursuit to make her testify, Section frames him for the murder of the rapist, Jack Crane. O'Brien is given the choice between recruitment and imprisonment. He chooses to join Section. O'Brien returns in S5 as Michael's "temporary" replacement. He is killed during a mission in S5 "The Man Behind The Curtain".