An Enterprise lieutenant sent with Sulu to scout Beta III in 2267, who is hunted by the lawgivers of Landru and absorbed into the Body.
Lieutenant O'Neil appears in the first-season episode The Return of the Archons. He and Sulu are on the planet in local dress, looking for traces of the starship Archon, lost a century before, when the robed lawgivers come after them. Sulu calls for beam-up. O'Neil panics and runs.
Sulu returns to the ship smiling and vacant, and Kirk takes a larger party down. They later find O'Neil in the same condition, a calm member of the Body, and Reger warns that Landru can see through his eyes. He is left behind under sedation.
When Kirk and Spock talk the Landru computer into destroying itself, its hold on the population ends and O'Neil recovers with the rest. He was played by Sean Morgan.