Chief Miles O'Brien (Colm Meaney) was a popular recurring character on Star Trek: The Next Generation, but he really came into his own as a series regular on Star Trek: Deep Space Nine. Premiering in 1992, DS9 was the first spinoff from TNG and it remains unique as the only Star Trek series set on a space station rather than a starship. After actress Michelle Forbes declined to play Ro Laren on DS9, Meaney became the only TNG actor to join the spinoff's main cast until Michael Dorn brought Worf to DS9 in season 4.

Miles Edward O'Brien was born on 2328 in Ireland, Earth; in 2345, the 17-year-old O'Brien enlisted in Starfleet, where he attended engineering school. He served aboard the U.S.S. Rutledge, among half a dozen other starships, before he joined the crew of the Federation flagship, Captain Jean-Luc Picard's (Patrick Stewart) U.S.S. Enterprise-D, as Chief Petty Officer. Colm Meaney was part of the TNG cast since the series premiere, "Encounter at Farpoint"; at first, TNG used Meaney's character as a utility person, placing him on the bridge at Security and sometimes at the helm, before O'Brien was finally stationed as transporter chief, which became his regular role. In the TNG season 4 episode "Data's Day," Miles married Keiko Ishikawa (Rosalind Chao), and their daughter Molly, who was delivered by Worf, was born in TNG season 5, which would be the O'Briens' final year as part of the TNG cast.

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Chief O'Brien truly shined as part of Star Trek: Deep Space Nine, where he was promoted to the station's Chief of Operations. In the early days, Miles was instrumental in moving the station from Bajor's orbit to guarding the Wormhole to the Gamma Quadrant. He was also responsible for the numerous repairs necessary to complete DS9's transition from a Cardassian station to a starbase controlled by Starfleet. Chief O'Brien appeared in all seven seasons of DS9, although he was killed in the season 3 episode "Visionary" and was replaced by a version of himself from several hours in the future. Still, O'Brien retrofitted the station's weapons array with 5000 photon torpedoes at the start of DS9 season 4, and he also mentored both Jake Sisko (Cirroc Lofton) and Nog (Aron Eisenberg), who joined Starfleet, in Engineering. O'Brien bravely fought in the Dominion War and he was part of every major victory (and defeat) Deep Space Nine saw during that devastating conflict.

Miles formed many lasting friendships on DS9, especially with Dr. Julian Bashir (Alexander Siddig), whom he grew extremely close to. After disliking each other at first, Miles and Julian became almost inseparable, whether they played darts together at Quark's Bar or cosplayed the Battle of Britain and defended the Alamo in the Holosuites. The O'Briens' son, Kirayoshi, was born in DS9 season 5, although an accident led to Dr. Bashir transferring the baby to Major Kira Nerys (Nana Visitor), who carried the infant to term. During Kira's pregnancy, she and Miles developed a mutual attraction, but they both decided it would be best to forget about it. Miles was also one of Worf's few close friends aboard DS9 and he played a ceremonial role at the Klingon's wedding to the late Jadzia Dax (Terry Farrell).

When the Dominion War was over, O'Brien accepted a position as Professor of Engineering at Starfleet Academy. Miles departed the station in the DS9 series finale, "What You Leave Behind," which was his last canonical appearance in Star Trek. In a cast that included a Captain who was half-Prophet, Bajorans, a shapeshifter, a genetically-engineered doctor, Trills, a Klingon, a shapeshifter, and various Cardassians and Ferengi, Chief O'Brien was beloved by Trekkers as the everyman character who was the most unapologetically and endearingly human person on Star Trek: Deep Space Nine.

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