Star Trek Canon Special Note
Not everything listed in the origin is Canon. That is because only television shows and movies are considered canon for Star Trek. This being a comic website there are many things here that are not canon. So unless it is contradicted in film it should be listed as part of the character biography.
Part of O'Brien's early career was in the Cardassian War aboard the U.S.S. Rutledge and it shaped him greatly into the man he became. Through a chance life and death situation he found that he had a skill for engineering. He also built a real dislike/hatred for the Cardassians. This dislike/hatred for Cardassians is something he struggled with for decades after the war. After the war O'Brien was stationed on the Flagship of the Federation and had a role in it's first mission and many more afterwards.
The character of O'Brien was created during the Star Trek: The next Generation Television show. His character however evolved very slowly unlike many other characters. He was first seen in the pilot episode of the show but had no name attributed to him. His rank and title changed as well as the series went on. By the fourth season however O'Brien had a name and some minor characteristics that were sprinkled about as the show had gone on. Most importantly was that he was a non-commissioned (non-com) officer.
Being a non-com O'Brien represented all the enlisted personal on the show. He was also a family man who actually was allowed to marry and have a family on the show and have them be a regular part of the series. This was especially true when the character became one of the main characters in the next Star Trek show, Star Trek: Deep Space Nine (DS9).
Since O'Brien wasn't originally suppose to be a main cast member he was allowed to grow as a character. In Next Generation he got married and had his first child. When DS9 started the whole family of characters became a part of the show.
Another part of his evolution that was not a smooth transfer was his rank. The show the Next Generation never really dealt with non commissioned crewmen and the rank insignia was not well established for these ranks. O'Brien even wore the rank of Lieutenant and was even called one on the show. Most the time however he was referred to as chief. It was not until about halfway through the Deep Space Nine series that his rank insignia was clearly established.
Once O'Brien transferred to the Enterprise his life really began to grow as he met and married Keiko. Miles and Keiko O'Brien also had their first child Molly O'Brien who was born during a crisis situation that resulted in the Tactical Officer Worf delivering the child. The new family was only together a short time before Miles was assigned to the O'Brien Space station.
Keiko was not real happy on DS9 and only found a purpose when she opened up a school. O'Brien himself had a hard time at first. The technology on the ship was part Cardassian, Bajoran and Federation so it was a very complicated system to work with. This stress however was really a labor of love leaving his true annoyance being Julian Bashir.
Miles had a real dislike for the green Bashir and did all he could to avoid him. The irony in this is they became the very best of friends and were virtually inseparable as they served together. Bahsir was there for Miles during his darkest times.
No time was darker for O'Brien than when he was sentence and convicted to a crime of espionage that he didn't even commit. Not only was he sentence but he served about twenty years in prison. What O'Brien did not know was that his incarceration was imposed upon him by an alien technology so that O'Brien did not age or even spend any real time in prison but his mind felt that he did. O'Brien couldn't even adjust to time on the station. He became haunted by the image of his friend and cell mate he had killed because he believed he was stealing food. He was so bothered by it that he tried to kill himself and it was Bashir who convinced him to live.
O'Brien and Keiko would go on and have another child and after the Dominion War, Miles would go back to planet earth to teach engineering at Starfleet. This was an odd move for him as he loved to work on challenging problems and seemed to have a need for action. It is likely that he will not be there long.