Jack O'Neill is a US air force officer. For most of his carer, he was the leader of SG-1 from its creation until his promotion to commander of Stargate command, and eventually to the department of home-world security. In that time, he has made first contact with countless of species and has uncovered new technologies to help defend the Earth against the Goa'uld, the Replicators, the Ori, and other threats to humanity.
O'Neill is a proficient pilot, with extensive training in special forces. He is notable for his sense of humor and general casual attitude. He is fond of the Simpsons, has a tendency not to lock his door, and can crack jokes in any occasion. He also has a great aversity to technical matters and science, repeatedly being baffled by Samantha Carter's explanations and knowledge.
Despite this, he is a very capable soldier and leader. His quick thinking has saved his team, and indeed, the Earth, on many occasions. It was also implied that he is a lot smarter than he wants people to think he is.
O'Neill is the first human discovered to possess and Ancient technology activation gen, allowing him to operate Ancient tech. He is quite proficient in this matter.
Jack O'Neill was at some point a special ops agent who had several missions in Irak during operation Desert Storm. At one point he married Sara O'Neill and had a son called Charlie. O'Neill loved his wife and son very much. Unfortunately, one day, Charlie found his father's gun and accidentally killed himself with it. O'Neill was devastated by the death of his son, resigning from the Air Force and even contemplating suicide.
1994 - Stargate program
Shortly after, O'Neill was recalled to active duty when Doctor Daniel Jackson managed to uncover the function of a highly advanced alien device called the Stargate. Activating the Stargate allowed them to establish a stable wormhole between Earth and the Planet Abydos. O'Neill was chosen to lead an expedition to Abydos and determine if any threat exists on the other side. If any threat exists, he was to detonate a nuclear device and destroy the Abydos Stargate. Daniel Jackson was also on this team.
Arriving on Abydos, O'Neill and his team uncovered a primitive settlement of ancient Egyptians, ruled by Ra, the supreme lord of a galaxy spanning tyrannical race known as the Goa'uld. Soon after their arrival, Ra himself came in his ship. He uncovered the explorers from Earth, and threatened to send the nuke, enhanced by his technology, back to Earth and destroy our civilization. O'Neill and his team managed to stop Ra, instead teleporting the nuke back to his ship and killing him.
With the threat from Ra gone, Daniel Jackson decided to remain on Abydos, and O'Neill and his team returned to Earth, telling everyone that they activated the nuke and destroyed the other stargate. The Stargate was mothballed, and O'Neill once again retired.
1997 - SG-1
O'Neill remained inactive for the next three years, his wife leaving him in the meantime. However, he was once again recalled when another Goa'uld, Apophis, came through the Stargate, killing three soldiers and kidnapping a female in search of a host for his new queen. O'Neill was forced to confess about the survival of Abydos and Daniel Jackson. He went back to Abydos to find Daniel Jackson, where they discovered that there are hundreds of Stargates throughout the Milky Way, employed by the Goa'uld as means of conquest and transportation.
With the threat of the Goa'uld recognized, Stargate Command was reformed under the command of General George Hammond, with SG-1, consisting of Jack O'Neill, Daniel Jackson, Captain Samantha Carter and a renegade Jaffa Teal'c, as the primary team.
SG-1 explored many worlds and encountered many civilizations ruled by the Goa'uld, while at the same time battling against the system lord Apophis. One time, O'Neill and Carter found themselves trapped on an ice covered planet with an inoperable Stargate (this was, in fact Antarctica). Dr. Jackson managed to deduce their location, which resulted in SG-1 finding the second gate buried on Earth.
SG-1 also had robotic copies of them made on an alien planet. These copies, not thinking they are the real thing, went back to SG-1 where they were promptly discovered. The real SG-1 was recovered from the planet, and the robots decided to bury their gate and stay on the planet.
Senator Kinsey attempted to shut down the Stargate program because of its high cost. However, learning that Apophis has launched two motherships on their way to Earth, SG-1 hijacked the Stargate, arriving on Apophis' Mothership and sabotaging them. This act saved Earth from destruction and ensured the continuation of the Stargate program.
O'Neill accidentally had the entire database of an advanced alien race known as the Ancients downloaded into his mind. This knowledge was slowly taking over his brain and killing him in the process. To survive, O'Neill subconscious mind engineered a sequence of events that lead him to Othala, home-world of the benevolent race the Asgard, who extracted the knowledge from his mind, and continued to hold him in high regard. this knowledge allowed O'Neill to program dozens of new Stargate addresses into the gate computer, and through that the team discovered that the Ancients were in fact the true builders of the Stargates.
SG-1 was later kidnapped by another Goa'uld called Hathor, who attempted to extract information from them through an elaborate hoax. This backfired, and O'Neill killed Hathor in the process. This once again brought Earth to the attention of the System Lords. To save Earth, the Asgard began negotiations to include Earth in the protected planets treaty. O'Neill represented Earth in those negotiations. They were successful, securing Earth's safety for several years to come.
SG-1 was on the planet Edora, when the Stargate was buried by a meteor shower. The rest of SG-1 managed to escape to Earth, but O'Neill remained stranded. He remained there for three months, until Major Carter built a particle beam that blasted through the layers of rock and allowed them to un-bury the gate.
O'Neill was also involved in uncovering a secret organization under command of Harry Mayborne that stole advanced technology from Earth's allies. To do this he stole a piece of Tollan tech, resulting in his discharge from the air force, and infiltration in the shadow operation. He was successful in uncovering and apprehending the criminals with Asgard help.
While on his way to vacation, O'Neill was beamed aboard Thor's ship, the Beliskner, in order to help Thor fight the enemy of the Asgard, mechanical bugs known as the Replicators. O'Neill and the rest of SG-1 managed to destroy the ship before it could land and infest the Earth. To escape, they beamed the Stargate up to the ship, which later fell to the bottom of the ocean, forcing SG to use the Antarctica gate from then on. One Replicator survived, however, and infested a Russian submarine. Jack and Teal'c boarded the sub in order to destroy the Replicators. They were overwhelmed, but were rescued by Thor at the moment the navy destroyed the infested sub.
O'Neill and the rest of SG-1 were also test subjects in a Tok'Ra experiment involving strength enhancing armbands. While the armbands enhanced all their natural abilities, they also made them highly unstable. They tried to single handedly destroy Apophis' new mother-ship. While they managed to set the ship to self destruct, they would themselves be destroyed if not for the timely intervention of Teal'c, whose Jaffa physiology negated the effects of the armbands.
When the Tau'ri and the Tok'Ra were about to sign an alliance, a threat posed by Zatarcs, brainwashed Goa'uld agents, became apparent. Both O'Neill and Carter were identified as zatarc due to inconsistencies in their memory. This was later revealed to be because they had suppressed feelings for each other. When they admitted this to each other, they were given a clean bill of health.
Some time later O'Neill and Teal'c were stuck in a time loop, reliving the same day over and over again, while at first determined to solve the problem as soon as possible, they eventually (realizing that anything they do would be undone) started doing a number crazy things, like riding a bike in the base, doing pottery, golfing through the gate and kissing Carter. The time loop was eventually undone.
In 2001 Jack was forced to briefly ally with former nemesis Harry Mayborne to uncover a plot by the NID and reinstate General Hammond as commander of SG, their investigation led them to senator Kinsey.
With the help of their robotic doubles, SG-1 managed to kill the Goa'uld system lord Cronus, commandeering his Ha'tak in the process. With this ship, they planed to destroy Apophis' fleet by plunging an active Stargate connected to a black hole into a star, creating a supernova which would wipe out the fleet. The plan backfired when both SG-1 and Apophis ended up in the Ida galaxy, where they were attacked by the Replicators. The replicators brought the ship back to Milky Way, but SG-1 sabotaged the engines, causing it to crash into the planet, creating a sizable boom. This also marked the final death of Apophis. However, Teal'c was brainwashed into thinking Apophis was indeed his God, and SG-1 had to submit him to an old ritual to break Apophis' hold on his mind.
On a mission to Langara, Daniel Jackson was exposed to a massive amount of radiation and was forced to ascend to another plane of existence, with help from an ethereal being, Oma Desala. SG-1 was left one member short. Jack was resistant to assigning a new member, not wanting to destabilize the friendship in the group. Eventually, under pressure from superiors, he chose Jonas Quinn from Langara, who was exiled from his home-world on the same mission. While apprehensive of him at first, Jack eventually learned to trust him. At that time the Tau'ri also became aware of a powerful ancient Goa'uld called Anubis, who was making a quick rise to power, and had acquired the technology of the Ancients to aid him.
Anubis attacked Earth with an Ancient Stargate destroying weapon, which fueled energy into the gate until it exploded. O'Neill managed to remove the gate from Earth by piloting an experimental craft X302 and sending the gate into Hyperspace. SG was from then on forced to use the original gate, which the Russians salvaged from the ocean. Anubis' weapon was subsequently destroyed.
After this SG-1 found a body frozen near the site where the antarctic Stargate was found. She appeared to human but possessed many advanced abilities like healing others. Unfortunately, she also carried a deadly plague, and soon everyone began to feel ill. Ayana, as she was named, managed to heal everyone, but died before she could help O'Neill. In order to save his life, O'Neill was forced to blend with a Tok'Ra symbiote. The blending was not successful, and the symbiote, influenced by O'Neill own personality, made an attempt to rescue a slave of the system lord Baal, whom he fell in love with (the slave, not Baal himself). He did not succeed, and O'Neill was left a prisoner of Baal. Baal tortured and killed O'Neill many times, each time reviving him with a sarcophagus. While the torture slowly drove O'Neill to madness, he was comforted by the ascended Daniel. Eventually Sam and Teal'c managed to give him a chance to escape, thought the experience left him with a deeper distrust of Tok'Ra, and a great hatred for Baal.
O'Neill was at one point accused of assassinating senator Kinsey, but he was in fact framed by a rogue NID operative with an alien holographic image device. He was exonerated with the help of SG-1.
After that Jack was approached by Mayborne once again, claiming to know the location of an Ancient weapons cache. In truth, he was looking for a utopian society of an alien race known as the Furlings. The society was already destroyed by the Goa'uld, and Jack and Harry were stuck on the planet for several weeks until SG figured out where they were.
Later, Daniel Jackson came back, informing them of Anubis' plan to assemble six devices known as the eyes of the Goa'uld into a power source for his super-weapons. SG-1 found one of the eyes, the eye of Ra, on Abydos, but were forced to give it up to Anubis when he threatened to destroy the planet. Anubis destroyed the planet anyway, but the people of Abydos were saved by Oma Desala. For his transgressions (namely interacting with the lower plane) Daniel was banished back to his mortal body, and rejoined SG-1. Jonas Quinn later came up with a plan to destroy Anubis' super-weapon, which Jack executed by shooting missiles into the power source of Anubis' mother-ship.
Anubis went to Langara in search of a new power source, but while there, he was attacked by the combined fleet of other system lords, destroying his ship and forcing him to retreat. This event allowed Jonas Quinn to return to Langara as a hero.
O'Neill was later cloned by a rogue member of the Asgard, but, due to a marker, placed by the Asgard high council in his DNA, the clone never reached maturity. This allowed SG to uncover the rogue Asgard and expose him to Thor. Jack's clone was enlisted into high school. They agreed not to stay in touch.
Anubis once again returned with intention of conquering the Earth. In order to stop him, O'Neill once again had the Ancient's knowledge downloaded into his mind. With this knowledge he was able to uncover an Ancient outpost on Antarctica, and activate the Ancients' drone weapons, which promptly devastated Anubis' fleet and ruined his plans of conquest. However, with the knowledge wreaking havoc on his system, O'Neill was forced to enter a stasis chamber to avoid imminent death.
He was saved by Thor, who used the information O'Neill still had in his brain to help devise an anti-Replicator weapon. the weapon was successful in disrupting the Replicators basic functions, allowing the Asgard to wipe out all replicators infesting their home-world.
Upon return to Earth, O'Neill was promoted to brigadier General, and given command of SGC in light of General Hammond's retiring. He was not fond of this, and frequently stated his frustration for not going through the gate anymore.
He did go on a mission again when a Goa'uld Ares threatened a planet where Harry Mayborne resided. There O'Neill activated an Ancient puddle jumper and used it to destroy Ares' Ha'tak.
O'Neill was also the one who authorized and sent away the Atlantis expedition to Pegasus galaxy.
O'Neill was promoted to major general, and was placed at the head of home-world security, giving the command of SGC to general Hank Landry.
Jack was called in when Daniel was found to be a Prior. After a heated debate, he was convinced to allow SG-1 to capture an Ori battle-cruiser carrying the incomplete Sangraal, hoping to send it to the Alteran Home Galaxy and neutralize the Ori. Daniel, however, commandeered the Odyssey and used Jack to fly him to the Super-gate, as part of his plan. O'Neill gave the order to Daedalus to destroy the Stargate linked to the Super-gate, allowing Daniel to dial the Super-gate and send the weapon through. The mission was a success, though it took further steps by the SG-1 to eliminate the threat of the Ori.