Sergeant O'Neill is a member of the 33rd Infantry Battalion that is mentioned in Spec Ops: The Line.
Background[]
O'Neill would seem to be one of the disciplinary officers within the 33rd Infantry, as his name can be seen on most of the soldier case files found throughout the game, listed as the Case Officer. What exactly his duties are are unknown.
In The Line[]
The only reference to O'Neill in-game is his name on several case files for missing 33rd Infantry soldiers found scattered throughout the game, mainly on the Highway, inside The Gate's command center, and at The Marina.