Given the fact that Jack O'Neill is one of the main characters on the show, we really don't know that much about him. He was born in Chicago, raised in Minnesota and either owns or has the rights to his grandfather's cabin in Minnesota.

He's career military and has likely been in the Air Force for twenty years or so. During that time he served in Black/Secret ops and we've been shown on the show that he's been a pilot and master parachutist.

He can be down right cranky and disrespectful at times, but also fiercely loyal to those he trusts. He doesn’t trust easily, but once you've earned his trust, you have it for life.

He's very cynical and not easily fooled, preferring to expect the worst and be pleasantly surprised by the worst not happening.

Because of this, some may see him as a defeatist but I tend to see him more as a realist. I think in the course of his career, he's had too many things go wrong to ever expect them to automatically go right.

 

Family

We don't really know much about Jack's family. There's never been a mention of his parents other than a grandfather who Jack inherited the cabin from.

He was married to Sara and they had one child, Charlie, who killed himself playing with Jack's gun. Their marriage is now dissolved and while Charlie has come up a time or two, Sara is rarely mentioned.

Jack has had the occasional romantic interest, some of which the result of intoxication but others totally voluntary. He doesn't seem to be the 'one night stand' kind of person, choosing to refrain from a relationship than have a casual fling.

He's also that way with friends. He does not tend to accept people easily, preferring to keep them at arms distance until he feels that he can trust them.

We've seen this with members of his team. Jack is rather distant and reserved until something happens and he decides to trust them and allow himself to open up to them.

To a certain extent, I think Jack sees friends as a vulnerability. If he's emotionally close to someone, then it's going to be harder to make life and death command decisions, which I think is one reason he tends to keep people at arms length.

Career

Jack's career is important to him, but it's not the sole driving force in his life. He's shown more than once that he's willing to abandon his career if he doesn’t like a situation. It's possible that Jack could receive a promotion to General upon his retirement, sort of a thank you for his years of service, but I think that's the only real way he'll get promoted.

His latent disrespect for authority has made him many enemies, and some of those enemies are in powerful places. Since promotion to general takes congressional approval, he may have a bit of a hard time getting it.

 

Future

Jack's future is rather up in the air. He has retired in the past and seems more than willing to do it again. I also think that he's facing physical issues, a bad knee and a reference to a back problem, not to mention recent emotional trauma. I also think that he's had a bit of a personal epiphany in recent years. After his divorce from Sara, I think he devoted himself to his job, likely to not have to deal with his personal issues. Now, I feel that he's learned that, while a job is important, he won't have it forever and unless he wants to end up all alone, he needs to start to give his personal life some priority. (And is it possible that Jack's outlook on life is mirroring RDA's with his devotion to his daughter?)

It is possible that Jack could stay in the service until he has to retire, which is after thirty years of service- about 10 more years or so. I do think, however, that if he stays in the service, he will take on a less active role, one with better hours and allowing more time to pursue a personal life.

If he does retire, I don't see him cutting himself off totally, at least not for a few more years. In 2010, he had totally distanced himself from the SGC, but he was also suffering from some intense hurt feelings and a sense of betrayal.

It has been suggested that Jack could be a viable replacement for Hammond when/if the general retires. Part of me can see this, but another part of me can't. He is the logical choice due to his familiarity with the program and especially his relationship with the
Asgard and other alien races, but he has one huge strike against him, his disregard for authority and impatience with bureaucracy. He doesn’t have a lot of political friends, and that could serve against him.

I can see Jack possibly serving some time as an advisor or even alien ambassador. His experience would be valuable to them and it would also allow him more personal time than active duty.

I do think that Jack is approaching the end of his career and I also think that his friendships/relationships are going to influence where he goes from here. In 2010, with a lack of friends and relationships, he was a withdrawn loner. If he keeps in contact with the SGC it will be because of the people there rather than any sense of duty. He'll stay for his friends.