Played by: Colm Meaney
Full name: Miles Edward O'Brien
Species: Human (Irish descent)
Position/occupation: Chief of operations, Deep Space Nine
Rank/title: Chief Petty Officer
Birthplace or childhood home: Earth (Ireland)
Parents: Michael (father); mother (died in 2368); stepmother
Siblings: two brothers
Children: Molly (daughter, born 2368); Kirayoshi (son, born 2373)
Other family: cousin (female); Sean Aloysius O'Brien (ancestor); King Brian Boru (alleged ancestor)
Hobbies/likes/interests: O'Brien enjoyed meat-and-potatoes types of cuisine, coffee, synthale, and (surprisingly) Federation combat rations. In his leisure hours, he was known to go kayaking and play darts; he also still played the cello (at least he did so while still on the Enterprise). He also owned several detective novels which he loaned to Odo. O'Brien played racquetball as well, and was a champion at one time.
Friends elsewhere: The Enterprise officers (Capt. Picard, Cmdr. Riker, Deanna Troi, Lt. Cmdr. Data, Lt. Cmdr. Geordi La Forge, Dr. Beverly Crusher)
Miles' father wanted him to be a cellist, and got him accepted to the Aldebaran Music Academy; but Miles signed up for Starfleet instead. He went the enlisted route, and served on 5 ships, including the Rutledge, prior to his time on the Enterprise. O'Brien built up a record that by 2372 would include 235 combat engagements, and 15 decorations by Starfleet.
(Note: Confusingly, in TNG, O'Brien was referred to as a lieutenant, meaning that he attended the Academy and was an officer; this makes some sense, considering that he was tactical officer on the Rutledge, yet on DS9 he indicated several times that he was an enlisted man.)
O'Brien was serving as tactical officer on the Rutledge during the Federation/Cardassian war when he participated in a rescue mission to Setlik III after a massacre there by the Cardassians. He killed a Cardassian there; the incident haunted him for a long time. O'Brien was eventually assigned to the Enterprise-D, first as battle bridge conn officer, and later as transporter chief. He married Keiko Ishikawa in the ship's Ten Forward lounge, where his daughter Molly was later born. O'Brien took a promotion and transfer to DS9 when a position as chief of operations opened up there, and brought his family, although Keiko's inability to pursue her career on the station caused some strife between them.
In "Captive Pursuit", O'Brien befriended an alien called Tosk, who was the prey in a ritual hunt, and broke the Prime Directive to help Tosk escape. He later found himself named as successor to the Sirah of a Bajoran village, which expected him to save them from destruction by the Dal'Rok ("The Storyteller"); and was plagued by the fairy-tale character Rumpelstiltskin ("If Wishes Were Horses"). He contended with the station computer when it suddenly began craving his attention ("The Forsaken"), and uncovered his assistant Neela's plot to assassinate Vedek Bareil in "In the Hands of the Prophets".
O'Brien aided Kira in rescuing Li Nalas ("The Homecoming"), and later that year, was maneuvered by Quark into playing a charity racquetball match against Bashir ("Rivals"). In "Armageddon Game", he and Bashir were reported killed while they were actually stranded and on the run from T'Lani and Kellerun officials who wanted them dead to eliminate all knowledge of a bio-weapon. O'Brien had been infected by the bio-weapon and was near death when they were rescued. He was later kidnapped by the Paradan government during a civil war, while a replicant took his place on DS9 ("Whispers"); and he was stranded with Sisko in a utopian colony which rejected technology ("Paradise"). In "Tribunal", O'Brien was arrested and tried on Cardassia Prime, on a trumped-up charge of running weapons to the Maquis, which turned out to be a plot by Central Command to "prove" that the Maquis had Starfleet's approval.
Seeing Keiko's depression when she closed her school due to lack of students, Miles tried to cheer her up, and finally suggested she go on an expedition on Bajor as a botanist ("The House of Quark"). They fought when she returned for the Gratitude Festival, but eventually made up ("Fascination"). In "Visionary", after suffering radiation poisoning, O'Brien traveled ahead five hours and back several times, due to an orbiting, cloaked Romulan warbird (with an artificial singularity powering its warp drive). He died after sending his future self back to warn the other officers and take his place.
O'Brien was captured by Jem'Hadar with Bashir in "Hippocratic Oath", and opposed Julian's work to cure the Jem'Hadar's need for Ketracel-white. Later that year, in "Accession", when Keiko returned from her expedition on Bajor, Miles learned she was pregnant with their second child, a son. In "Hard Time", O'Brien was convicted of espionage on Argratha, and punished with an interactive program that gave him the mental experience of twenty years in prison. When he came back to the station, O'Brien had difficulties readjusting, and had visions of his cellmate Ee'char, whom he had killed for some scraps of food. He nearly committed suicide, but was talked out of it by Bashir.
After trying to keep his wounded subordinate Muniz alive, to no avail, O'Brien mourned his death in "The Ship". Then he was briefly drawn to Kira (who was carrying his baby as a surrogate) in "Looking for Par'Mach in All the Wrong Places". In "The Assignment", a pah-wraith took Keiko's body hostage to force Miles to aid it in destroying the wormhole aliens; Miles managed to destroy the wraith instead. Then O'Brien finally witnessed the birth of his son Kirayoshi in "The Begotten".
O'Brien was pitted against a homicidal Garak on an abandoned Cardassian station in "Empok Nor", and was one of the Starfleet personnel who left the station in "Call to Arms" and returned in "Favor the Bold"/"Sacrifice of Angels". In "Honor Among Thieves", O'Brien worked for Starfleet Intelligence, and befriended a man named Bilby in order to get information on the Orion Syndicate's Starfleet connection. He finally revealed himself to Bilby and warned him of the setup by Starfleet Intelligence and the Klingons, but Bilby went to his death anyway. Afterwards, O'Brien adopted Bilby's cat Chester, and kept in touch with his widow, Morica. Keiko and the children returned to DS9 after having been sent to Earth before the Dominion invasion. During a family picnic, Molly fell into a time portal and re-emerged as an 18-year-old savage. Her parents tried to resocialize her, but under the threat of her being institutionalized, they chose to let her go back into the past. However, after she had gone back through the portal, she reappeared as an 8-year-old again ("Time's Orphan").
O'Brien learned an interesting lesson in "navigating" the "Great Material Continuum" from Nog in "Treachery, Faith and the Great River". Later, when Morica Bilby disappeared, O'Brien secretly went to New Sydney to look for her. He eventually found her, dead in a river, and learned that Ezri's brother Norvo was responsible ("Prodigal Daughter"). O'Brien decided to go with Bashir up against Section 31 to find a cure for the Founders' disease that was now afflicting Odo ("When It Rains..."), and it was his idea for Bashir to lure a member of Section 31 to DS9 by claiming to have found a cure ("Tacking Into the Wind"). He accompanied Bashir into Sloan's mind to search for the cure, which they eventually found ("Extreme Measures").
In "What You Leave Behind", O'Brien had accepted an offer to return to Earth and become a professor of engineering at Starfleet Academy. After the final victory over the Dominion, he left the station with his family.
Other facts: O'Brien once had arachnophobia, but overcame it (it is unknown what became of his pet Lycosa tarantula, Christina, seen in "Realm of Fear" [TNG]). O'Brien acquired a cat named Chester in "Honor Among Thieves".