As the commanding officer of the SG-1 team, Colonel O'Neill's assignment is to defend Earth from possible hostile invasion, and to explore new worlds and return new technologies to Earth for study and further development. Jack joined the Air Force at 18 years of age. After proving to be a great airman, he was assigned to a special training program for covert operations/infiltration. He resigned from service when his son Charlie died by accidentally shooting himself with O'Neill's gun. O'Neill never recovered from the loss, which drove his wife to eventually leave him. He was recalled by his former commanding officer to go through the Stargate on the first mission precisely because of his willingness to die. As a result from this mission, Jack gained a desire to actually live.

                    Jack has a wry, sardonic sense of humor. He is funny and not afraid to admit if he doesn't know something. O'Neill is an excellent team leader of SG-1 and a very good soldier. Jack takes care of his team, is not scared of taking risks, and everyone of SGC respects him greatly. General Hammond has complete confidence in him and often offers him a degree of leeway in his execution of his orders. They've contacted lots of alien races, and the Asgard are particularly taken with him and use him as their liaison with Earth. He has problems with scientists, finds them boring. This is why, initially, he thought Daniel was a geek and was a bit hard on Sam. Jack now likes and respects both of them. Although Jack cares about Sam 'more than he should' (translation- he is in love with her!). However when they start giving him long scientific explanation he tends to say, 'Just tell me if we can do it or not', or similar.