(coexisting on another dimensional plane)
Miles "Smiley" O'Brien was a Terran slave who worked for the Klingon-Cardassian Alliance aboard station Terok Nor, but later became the leader of the Terran Resistance Forces who fought the Terran Rebellion.
In 2370, O'Brien was working in the station's ore processing center, and had the slave classification "theta", which was a relatively respected and privileged position. O'Brien was also occasionally recruited to repair Benjamin Sisko's raider when it was docked at the station. Sisko claimed he hated the name Miles, and instead nicknamed O'Brien "Smiley", despite him not liking it. He had a good reputation on the station for being an excellent "tinkerer and putterer".
When Smiley met a Julian Bashir from another universe in the ore processing center, he initially thought that Bashir's "wild" tales of his counterpart being Chief of Operations was simply a ruse to get him to help Bashir. However, Bashir's earnest attitude won him over, along with the realization that anything had to be better than his slavery to the Alliance. Smiley helped Bashir try to escape from the station, on the condition that he be taken along to the other side.
However, when Captain Sisko also rejected Intendant Kira Nerys's authority, Smiley changed his mind and decided to join Sisko's crew, deciding that there was something worth staying for after all. (DS9: "Crossover")
When Captain Sisko was killed by the Alliance in early 2371, Smiley took charge of the Terran Rebellion. In order to complete Sisko's last mission, he recruited Benjamin Sisko from the other universe to take the place of his deceased counterpart long enough to convince Jennifer Sisko to leave the Alliance and start working for the Rebellion. (DS9: "Through the Looking Glass")
While he was aboard Deep Space 9, Smiley took the opportunity to download a large portion of the station's database, including the schematics for the USS Defiant. This information came in handy the next year, when the rebels managed to capture Terok Nor from the Alliance. Smiley used the stolen specifications to build the rebels' own warship, which they also christened Defiant. However, with the Alliance about to launch a counterattack to recapture the station, Smiley once again recruited Sisko in order to help them finish construction of the Defiant in time.
Smiley piloted the ISS Defiant during the Battle of Terok Nor, although he was replaced by Sisko partway through, because Sisko had greater experience flying the little warship. Following the successful conclusion of the battle, Smiley sent Sisko back home. (DS9: "Shattered Mirror")
In 2375, Smiley was still in command of the rebellion on Terok Nor. He commanded the Defiant when it pursued Brunt's ship as it stole a cloaking device from the station, and later negotiated Regent Worf's surrender and capture. (DS9: "The Emperor's New Cloak")
Prior to the establishment of the history of the televised mirror universe, a mirror version of O'Brien appeared in the novel Dark Mirror, depicting a mirror universe in which the Terran Empire had survived well into the 24th century. That version of O'Brien served as a conn officer aboard the ISS Enterprise (ICC-1701-D) from 2364 until at least 2367.
During the Star Trek Online mission "Crack in the Mirror", it is revealed that O'Brien had a son named James who murdered him sometime after 2375.