Lieutenant O'Neil was a male Human Starfleet officer in the 23rd century. He served in the operations division aboard the USS Enterprise in 2267 and 2268 under the command of Captain James T. Kirk, during the historic five-year mission.
In 2267, he was one of two men assigned to investigate the planet Beta III for clues to the disappearance of the Archon, a starship that vanished there in 2167. A wardrobe error left the landing party obvious to the sinister lawgivers, who enforced the will of the planet's leader, Landru. Hounded relentlessly by lawgivers, the landing party requested beam-up, but O'Neil panicked and fled the transporter coordinates. At some subsequent point, he was absorbed into the Body.
The second landing party encountered him in an alley, where he was one member of a crowd they were forced to stun. They took him with them, keeping him sedated, until they could determine what was wrong with him. Reger, a Beta III native who had befriended the landing party, advised against this, warning that Landru could find them through O'Neil. O'Neil later returned to normal when Landru destroyed itself. (TOS: "The Return of the Archons")
In 2268, O'Neil served as the transporter chief when the landing party beamed over to the USS Defiant. He attempted to repair the transporter following the attack by the Tholians, but ultimately required Scotty's assistance. (TOS: "The Tholian Web")
The first draft script of The Return of the Archons calls him Lieutenant Agnon.
In the novelization of The Return of the Archons, his name was spelled O'Neill. This spelling is also used in IDW's The Return of the Archons, Part 1 comic book reimagining.