Miles Edward O'Brien is a character in the fictional Star Trek universe, played by Colm Meaney. He was a secondary recurring character as a transporter chief on the USS Enterprise (NCC-1701-D) on Star Trek: The Next Generation, and a primary character as Deep Space Nine's chief of operations on Star Trek: Deep Space Nine.
Spoiler warning: Plot or ending details follow.
Miles O'Brien was born in Killarney, Ireland on September 2328. His father wanted him to be a cellist. His audition tape was sent to the El-Dabran Music Academy and he was accepted, but a few days before he was scheduled to start classes there, he joined Starfleet.
O'Brien participated in war against the Cardassians in the massacre of Setlik III, when he served aboard the USS Rutledge, commanded by Captain Benjamin Maxwell. After various assignments he was posted as transporter chief to the USS Enterprise, commanded by Captain Jean-Luc Picard. Miles married Keiko Ishikawa aboard the USS Enterprise in the TNG episode "Data's Day". She bore him a daughter, Molly (TNG episode "Disaster"), and after transferring to Deep Space Nine, commanded by Commander Benjamin Sisko, Keiko bore Miles a son, Kirayoshi, through a surrogate pregnancy by their crewmate Major Kira. Chief O'Brien also served aboard the USS Defiant under Captain Sisko and Lt. Commander Worf.
Miles O'Brien exists in the mirror universe first discovered by James T. Kirk in the classic episode "Mirror, Mirror". In the DS9 episode "Crossover" Dr. Julian Bashir encounters the mirror O'Brien as a Terran slave working the ore processing machines at Terok Nor. The mirror Sisko did not like the name "Miles" so he calls the mirror O'Brien "Smiley".
In the episode "Visionary", O'Brien had an accident in which he was poisoned by radiation and experienced several time shifts, each five hours into the future, witnessing a bar brawl, his own death twice, and the destruction of the station. At the end of the episode, he deliberately subjected himself to more radiation in order to time shift to the future to see exactly what caused the destruction. But the radiation was too much, and the past O'Brien died, while the future O'Brien went back in time to tell Sisko that there a cloaked Romulan Warbird circling the station, plotting to destroy it.
At the end of Star Trek: Deep Space Nine, O'Brien moves back to Earth to become a professor of engineering at Starfleet Academy.
During his appearances on TNG, he was shown as being of officer rank, initially an ensign, subsequently a lieutenant. However, in the episode "Family", Worf's father Sergey Rozhenko called him a "chief petty officer". O'Brien did not object to this, although he could have held his tongue in politeness.
On DS9, he was presented (and explicitly described himself) as being of enlisted rank. If he was indeed a Chief Petty Officer at the time of the meeting with Sergey, it is likely that he could have been promoted to a higher enlisted rank upon transfer to Deep Space Nine.