Miles Edward O'Brien was born in Ireland, Earth in September 2328.

In 2346, at the age of 18, O'Brien joined Starfleet as an enlisted crewman. His father had wanted Miles to take up a place at the Aldebaran Music Academy and become a concert cellist, but he chose a life in Starfleet instead.

During the Earth Cardassian War (promoted) he was assigned to the USS Rutledge, under the command of Benjamin Maxwell, where he served as junior tactical officer A year later, O'Brien was present at the aftermath of the Setlik III massacre, where his technical skill was useful in repairing a field transporter, allowing thirteen other Starfleet personnel and himself to escape a Cardassian patrol. This earned him a promotion to primary tactical officer.

O'Brien also served as part of a squad to aid the survivors of the massacre as Cardassians were still patrolling the settlement; he helped a group of women and children but was ambushed by two Cardassians. He was able to incapacitate one of them but he got into a fight with the other and accidentally killed his assailant when a Phaser passed to him was set to maximum. Until then, O'Brien had never killed anything or anyone (he was even worried about swatting mosquitoes as a child) and the incident left him with a deep-seated resentment towards Cardassians, bordering on outright racism on occasion. He often referred to them by the derogatory term "Cardies".

(Data gleaned from DS9: "Emissary", "Captive Pursuit", "The Homecoming", "The Siege", "Tribunal")

Following his service during the Federation-Cardassian War, he was stationed aboard the USS Enterprise-D and served as transporter chief for several years.
O'Brien transferred to the USS Enterprise-D in 2364, under the command of Captain Jean-Luc Picard. During the Farpoint Mission he was assigned as relief flight controller. He also stood duty in the security department, but was most often seen filling duty as transporter chief, where he was involved in, among other events, the rescue of Captain Picard from the Borg in 2367.

In all episodes of TNG Seasons 2-5, O'Brien is seen wearing the insignia of a full lieutenant while working as transporter chief; in one of them, "Where Silence Has Lease", he is also addressed as "lieutenant".

(TNG: "Encounter at Farpoint", "All Good Things...", "Lonely Among Us", "The Best of Both Worlds, Part II")

In 2367, Miles was forced to confront his former commanding officer, when Captain Maxwell went rogue, launching attacks on Cardassian outposts and ships in spite of a peace treaty between the Federation and Cardassia. It was O'Brien's knowledge of transporter systems that allowed him to beam aboard Maxwell's ship, the Phoenix, while her shields were up, giving him a chance to reason with Maxwell, ultimately preventing further bloodshed.

(TNG: "The Wounded")

During the Klingon Civil War, O'Brien also made use of his training as a tactical officer, following Lieutenant Worf's resignation from Starfleet. As a result, he was heavily involved in the deployment and maintenance of the tachyon detection grid that ultimately decided the conflict.

(TNG: "Redemption II")

In 2368, Miles, along with Deanna Troi and Data, was taken over by an alien life-form, which forced them to take hostages to use as a bargaining chip in negotiating with Picard. Among the prisoners were his wife Keiko O'Brien and their daughter Molly. Eventually, the life-form left when it was discovered that the moon was actually a 'penal colony' and that this was an attempt to escape.

(TNG: "Power Play")

He later jumped at the opportunity when he was offered a promotion to Chief of Operations aboard starbase Deep Space 9, although the switch did not sit as well with his wife Keiko. Together, Miles and Keiko had two children, Molly and Kirayoshi. The Dominion War nearly tore the family apart, however, and when it had finished, Miles was happy to accept a teaching position at Starfleet Academy on Earth.

For details of his service aboard Deep Space Nine click here

For details of his relationship and marriage to Keiko see Keiko O'briens Biograhy