It is not known for how long O'Brian has been in the employ of Stark International, but it is clear that his loyalties lie with Stane rather than the Stark family, as shown by his personal antipathy to Tony. Although for most of his appearances he does not wear a powered armor suit of his own. He once piloted the Crimson Dynamo armor and later on the newly created Iron Monger mech to tear down a derelict city block. When he disobeyed Stane's orders to continue demolition of a building (there were civilians in the demolitions zone), he was fired by Stane for his act of mercy.
It seems he was rehired in Iron Monger Lives, when he got Howard Stark to safety. In the episode Whitney Stane briefly disguises herself as O'Brien to wreak revenge on Tony, whom she holds responsible for her father's condition
- O'Brian is based on the Marvel Comics' Guardsman character, personified by Kevin and Michael O'Brien. In addition, as the security chief of Stark International he fulfills the function of his mainstream namesake at its most mundane level.