|First Appearance||Sanctuary (Part 1)|
|Last Appearance||Sanctuary (Part 2)|
|Allegiance||2nd Massachusetts Militia Regiment|
|Cause of Death||Killed by Terry Clayton.|
|O'Brian dead on the ground.|
|“||And they're going to have a security detachment with them, O'Brain and Fuller.||”|
— Tom to Frank.
O'Brian was a fighter in the 2nd Mass. O'Brian was a trusted fighter that was sent along with Fuller to escort the kids to the Sanctuary.
O'Brain helps secure the school from the said skitter and mech attack by flipping cars as barricades at the front of the school. was one of the fighters who Tom Mason selected to provide a security escort for the children of the 2nd Mass to make it to the 7th Mass base, along with Fuller, Terry Clayton and Mike. Later that day the group arrived at the ranch near sunset with no trouble, with Fuller and O'Brain leading the back of the group.
The next morning O'Brian and Fuller were supposed to be escorted back to the main road to get back to the 2nd Mass base, but both were leaded into a trap and both were shot in the heart by Clayton, and had there weapons and supplies taken. Later after Pope escaped the 7th Mass, he came across O'Brian's and Fuller's bodies.
|Season 1 appearances|
|Live and Learn||The Armory||Prisoner of War||Grace||Silent Kill|
|Sanctuary (Part 1)||Sanctuary (Part 2)||What Hides Beneath||Mutiny||Eight Hours|