Jonathan "Jack" O'Neill (seasons 1–8 main, seasons 9–10 recurring) – A United States Air Force Colonel and special operations veteran who led the original mission through the Stargate in Stargate (where he was played by Kurt Russell).

He is coaxed out of retirement in the pilot episode and serves as the leader of the SG-1 team in the first seven seasons. He takes charge of Stargate Command (SGC) after his promotion to Brigadier General at the beginning of season 8.

The series repeatedly alludes to romantic feelings between O'Neill and his second-in-command Carter, but the relationship is never shown as consummated outside of alternate reality scenarios; however Season 9 Episode 7 alludes to their forming a relationship. O'Neill is reassigned to Washington, D.C. before season 9 and receives a promotion to Major General.

He appears in a recurring role in seasons 9 and 10 of Stargate SG-1, as well as in Stargate: Continuum and in seasons 1 and 3 of Stargate Atlantis. O'Neill appears as a Lieutenant General in multiple episodes of Stargate Universe. A third direct-to-DVD SG-1 film titled Stargate: Revolution will center on O'Neill.