Notes : Chief Petty Officer Miles O'Brien is one of the most experienced of Deep Space Nine's crew, with a a Starfleet career spanning almost thirty years. He joined as an enlisted man at the age of eighteen, and served on board the USS Rutledge while that ship was involved in the Setlik massacre. Whilst on Setlik, O'Brien was in a room full of survivors when a Cardassian soldier entered. A woman in the room threw him a phaser which he fired at the Cardassian; unfortunately, the phaser had been set to kill and O'Brien vaporized the man. Up to this time O'Brien had never purposely harmed anything - he was even squeamish about killing insects - and the trauma of the event left him with a deep-seated dislike of the Cardassians who had forced him to become a killer. O'Brien would later begin to overcome his hatreds when he worked with several Cardassians whilst on the Enterprise and Deep Space Nine.

O'Briens best friend, Raymond Boone, was captured by the Cardassians during Setlik. Boone was held for a total of fifteen years before being released; but unknown to Starfleet, the Boone who was released was a Cardassian extensively altered to resemble a human and given a false identity. Boone later joined the Maquis, and framed O'Brien for the theft of photon warheads from DS9. O'Brien was convicted of the crime under the Cardassian legal system, but was released when Commander Sisko threatened to reveal Boones true identity.

O'Brien was killed in 2371 when a bizarre set of circumstances caused him to repeatedly jump through time. Another, apparently identical O'Brien from an alternate timeline took his place and has continued to serve on the station ever since.

O'Brien is one of the few Starfleet personnel to maintain a successful marriage, although his relationship with Keiko has often been strained badly by the pressures of living and working on Deep Space Nine. Keiko eventually accepted a position on a botanical survey team on Bajor, and the two lived apart for some time.

Miles' second child was almost killed before birth when Keiko suffered an accident whilst in a Runabout. Doctor Bashir was able to save the child by beaming it to Major Kira's womb, where it gestated before being born in 2373. In 2374 O'Brien was holidaying with his family on the planet Golana when his daughter Molly fell through an ancient time portal. He was able to get her back through the portal, but inaccuracies in the device meant that the Molly who returned was ten years older. Molly found it impossible to readjust to life on the station, so O'Brien returned her to the portal and sent her back through again. She emerged just an instant after her earlier self arrived, and sent the child straight back through. The original version of Molly was thus returned, but the older version erased herself from history with this action.

In 2375 Miles was offered a teaching post at Starfleet Academy once the war ended. He accepted, and soon after the final battle of the war the O'Brien family left Deep Space Nine for good.