Jack O'Neill (spelled "O'Neil" in the film) is a United States Air Force Colonel with experience in special ops. In the Stargate film, O'Neil is married to Sarah O'Neil and suffers a period of suicidal depression after his son accidentally shoots himself with O'Neil's personal sidearm. When Dr. Daniel Jackson deciphers the Stargate, O'Neil is recalled to active duty and leads a reconnaissance mission of soldiers and Daniel to the planet Abydos. O'Neil's standing order is to detonate a nuclear warhead near the Stargate at the sign of danger, but a young Abydonian boy named Skaara gives him a renewed sense of life. After the defeat of the local villain Ra, O'Neil and his team return to Earth while Daniel remains on Abydos with his new-found love.
With an incursion by the Goa'uld Apophis and the subsequent creation of Stargate Command (SGC) in the pilot episode of Stargate SG-1, O'Neill is given command of the primary SG team, SG-1, which includes Daniel Jackson, Captain Samantha Carter and the alien Teal'c. O'Neill is no longer married. In the season 2's , O'Neill has the Repository of the Ancients temporarily "downloaded" to his brain and becomes the first modern human to travel to another galaxy, the Asgard home galaxy. A self-sacrificing download of the Ancients' knowledge into his brain nearly kills O'Neill in , the season 7 finale. After the discovery of an Ancient outpost in Antarctica in the same episode, O'Neill operates an Ancient control chair, meaning that O'Neill possesses the ATA gene.
In recognition of his service and unique diplomatic position, O'Neill is promoted to Brigadier General in the season opener of season 8 and is given command of the Stargate Command. O'Neill has difficulty adjusting to his desktop new job at first. No longer a main character beginning with season 9, O'Neill becomes the new head of the Department of Homeworld Security and appears in two episodes early in season 9 while Major General Hank Landry (played by Beau Bridges) settles in as the new commander of the SGC. 'O'Neill also appears in two episodes of season 10 – in the milestone episode , and helping to save Daniel Jackson in . In his four appearances in Stargate Atlantis, O'Neill sends off the Atlantis Expedition to the Ancient city of Atlantis, is part of Elizabeth Weir's hallucination, and travels to Atlantis with Richard Woolsey for an alien negotiation in a two-part episode. O'Neill returns in the direct-to-DVD film Stargate Continuum in a supporting role; he is killed in the opening scenes but appears as an alternate version in another timeline and is subsequently rescued by the three remaining SG-1 members from the original timeline. O'Neill is expected to return in the third post-SG-1 film.
O'Neill is somewhat of a wit throughout the series. Enemies and allies alike are subjected to frequent quips and facetious remarks, which can at times have at least minor diplomatic consequences. He often derides Carter's scientific and Jackson's esoteric interests, though it is frequently hinted that he finds intellectuals intimidating. He is irreverent towards authorities, including his own superiors, and especially revels in mocking the System Lords, whom he views as overly theatrical and egotistical (as well as over dressed), which typically enrages them. It is implied that his more serious motivation for doing this with the System Lords in particular is to remind them that they are, in fact, not Gods as they insist on claiming to be. He is very protective of his friends, and has proved to be a powerful ally to General Landry and Doctor Weir in their confrontations with the IOA, as he is the only one who doesn't answer to them.
He also commonly corrects improper grammar, most often saying, "It's whom!", usually doing this when the correctee has SG-1 at a disadvantage, as in "The Other Guys". As a last resort, and sometimes as a first, he will use sarcasm in a confrontation. During the later seasons, he particularly enjoys mocking Ba'al on the basis of his name and his theatricality, often insulting him to his face. Following his departure from the regular cast of SG-1 in Season 9, his role as the witty personality seems to have been partially filled by Vala Mal Doran and distributed among the other cast members.
Throughout the series, O'Neill's relationship with Samantha Carter has been subjected to a great deal of interest and speculation. On two separate occasions, either O'Neill or a member of his team come into contact with (different) alternate realities in which O'Neill and Carter were either engaged or married. In the fourth season episode "Divide and Conquer", both O'Neill and Carter had to officially admit that their feelings for one another were more than professional. There are several events and incidents spread throughout the series that imply that the feelings they have for one another remain a subtle constant in their lives (shown in Season Four's episode "Window of Opportunity" when in one of his time loops he resigns his commission to the Air Force to kiss Sam), despite various outside influences, such as Carter's temporary fiancé Pete Shanahan, and O'Neill's brief relationship with Kerry Johnson, an officer of the CIA, as well as their relationships with offworlders, particularly (in Carter's case), Narim and Martouf.
Despite being capable of a high degree of compassion, including a great willingness for self-sacrifice when it appears necessary, he will generally advocate a military solution to problems in favour of other answers, and is often in conflict with other characters (usually Daniel Jackson) who prefer a more diplomatic or peaceful approach. Arrogance is also an Achilles' heel for him; O'Neill tends to display a characteristic of believing very strongly in his own moral superiority, and will at times not refrain from attempting to impose his own beliefs upon alien cultures discovered through the Stargate. There were at least a few occasions where this behaviour produced serious consequences. In the end, O'Neill does what he believes is the right thing to do, and is not unwilling to change his course of action if he finds that it is the wrong one.
Unlike Anderson's other famous on-screen persona, Angus MacGyver, O'Neill has no aversion to firearms, apart from the fact that his son, Charlie, accidentally shot himself with O'Neill's pistol and died (O'Neill never forgave himself). He is a career special operations operative, owns at least two personal firearms, and becomes extremely agitated whenever it is suggested he and his team go anywhere unarmed, including places known to be safe for SG-1 and where firearms may be a liability. Another trait that sets Anderson's two characters apart is that MacGyver was an all-around scientific genius; O'Neill, while far from stupid, has a fairly native form of intelligence which is almost exclusively militarily oriented, and prefers to skip over almost any scientific explanation (in which Carter and Daniel often indulge). He often, though half-jokingly, suggests "magnets" as the explanation for any technology that is beyond his understanding (magnets were the prime technology for Anderson's previous character, Nicodemus Legend). The primary exception to this is in the field of astronomy, as O'Neill is himself an amateur astronomer. Despite these differences, O'Neill and MacGyver do share some similarities, both play hockey, but also in their past: both characters served in the United States Special Forces.
Richard Dean Anderson was part of the main cast from season 1 through 8. He recurred for several episodes each season thereafter.
Richard Dean Anderson is expected to return in the third post-SG-1 movie and potentially more thereafter. Executive producer Brad Wright stressed the importance of O'Neill's presence in the Stargate Universe even after the character went on hiatus during the last two seasons of SG-1 when Anderson took a leave from regular acting.