O'Neill was born in Chicago, and raised in Minnesota. He has made many enemies in his years of service, and arguably fewer friends. However, his character has nobility that many of those encountered lack. Beyond the death of Charlie, very little is known about O'Neill's early years. He possess a sharp, sarcastic sense of humor, and is not above taunting an enemy in the heat of battle, or while in captivity.

If you count the skull fractures, Jack has had nine broken bones. He was parachuting between the borders of Iran and Iraq in the 1980s and hit the ground hard. There was no rescue. Instead, he spent nine days making it out on his own, back into allied territory. Jack spent several years in a black-ops unit of the U.S. military, and was imprisoned in Iraq for four months after a mission under the command of Frank Cromwell went bad.

O'Neill is forced to relive a failed black-ops mission. From "The Gamekeeper"

With this in mind, it can be deduced that O'Neill married before 1988. He confirmed he and wife Sarah were together at the time of his imprisonment, since she was his primary motivation for escaping. Sometime after escaping the prison, Charlie O'Neill was conceived.

Close relationships seem to be difficult for Jack. Major Charles Kawalsky was a friend of O'Neill, despite the fact he never knew Jack had a son. Kawalsky claimed he could beat jack at any game of street hockey. They had served together on at least one SWAT mission in East Germany, in 1982. Their mission was to infiltrate a wooded compound and capture a Russian agent; but enemy soliders were waiting, and O'Neill's friend and commanding officer, John, was killed.

A photo of Jack and his son, Charlie. From "Cold Lazarus"

During his off hours Jack spent time with his wife and son. Family brought terrific meaning to his life. Simple games of catch and baseball, his son's favorites, fulfilled the parts of his existence that were inaccessible through his military career. But one fateful autumn day, on the day Charlie came home from school with his class pictures, the boy uncovered his father's handgun and fatally shot himself inside their home. Jack and Sarah heard the shot from outside the house, and rushed Charlie to the hospital. The child did not survive.

The impact of Charlie's death sent O'Neill into deep depression. He was deactivated from duty until two of General West's officers arrived at his house, informing him that he had been brought back into active service at the Creek Mountain facility. The top-secret Stargate project was approaching a critical juncture. O'Neill accepted the mission that, if Dr. Daniel Jackson succeeded in opening this "door to heaven," he would go through to the other side to assess the situation and, if he found any hostile threats to Earth, would give his life to destroy the Stargate with a nuclear weapon.

Sarah and Jack hear the gunshot that would destroy their life. From "Cold Lazarus"

The Stargate was opened around 1996, forever changing O'Neill's life. On the other side of the portal was the Supreme System Lord Ra, who had seeded the world of Abydos with humans. With the help of Jackson and the native Abydonians, O'Neill instead used the bomb to obliterate the false god in orbit aboard his mothership. O'Neill returned to Earth, leaving Jackson, per his request, on Abydos.

Arriving home from the mission, O'Neill found Sarah wished him gone from their home. Soon divorced, he was alone for the first time in over a decade. With the Stargate program shut down and transplanted to Cheyenne Mountain, Jack retired from service. He quit smoking and spent much of his time in solitude, some on his roof staring at the stars through a telescope. It was there on his roof, one year later, that he learned that the Stargate had been reopened and a new threat had emerged.

With the inception of Stargate Command under Major General George Hammond, O'Neill agreed to come out of retirement to command the flagship field unit, SG-1. Along with Samantha Carter, his new second-in-command, he returned to Abydos to retrieve Dr. Jackson and uncover the source of the new Goa'uld threat. But his good friend Skaara, an Abydonian boy, was soon kidnapped by the Goa'uld System Lord Apophis, giving O'Neill a very personal reason to seek out and fight this new enemy.

Jack makes his feelings for Sam known. From "Window of Opportunity"

The SG-1 team and the worlds they have explored have forced O'Neill to reassess his future and his relationships. His initial dislike for Jackson changed to a fondness over the course of many years. Frequently disagreeing with the scientist, he found common ground nonetheless, realizing that both complemented each other's strengths, both in diplomacy and in battle. When Jackson died and ascended to a higher plane of existence, O'Neill struggled with how to deal with his grief -- but was eventually reunited with his lost friend. Jack also became a good friend and brother of Teal'c, a warrior as much as he, whom he convinced to turn against the Goa'uld and join them.

Hammond, despite being his commanding officer, has also become a friend and confidant. Frequently putting up with Jack's jagged-edged and sarcastic attitude, he calms the storms of frustration within the man in any decision he is forced to make. And beyond his friendship forged with Samantha Carter, O'Neill has confessed that he has come to care about her a lot more than their military relationship allows him to. Regulations prevent them from initiating a romantic relationship.

Among his many achievements over the years, O'Neill has been implanted with a symbiote twice, one a Goa'uld and one a Tok'ra named Kanan. He has had the Ancients' library downloaded into his brain. He kidnapped an alien child to allow her to experience life as a kid. He died and was revived countless times in Baal's sarcophagus. He has spent three months becoming part of Edoran society, believing he was fated to live out his life there. He was framed for the murder of a U.S. senator, rapidly aged by Goa'uld nanocites, possessed by a microscopic alien race, trapped in a time loop, and saved Earth more times than he can count.

O'Neill is murdered by Baal and repeatedly revived in a sarcophagus. From "Abyss"

He has also directly contributed to the defeat of such powerful Goa'uld as Ra, Apophis, Cronus, Seth, Hathor, Sokar, Nirrti, Heru-ur and Marduk, among others.

Many of Jack's dearest friends have not necessarily been human. Thor, the supreme commander of the Asgard fleet (whom Jack knows as his "buddy"), has been a friend from the beginning. The Asgard saw the potential of the human race in Jack and chose to ally themselves with Earth. Thor has even revealed that the Asgard have found the colonel to be quite unique -- the next stage in human evolution.