A replicant of Miles O'Brien was created in 2370 by officials of the Parada government after abducting the real O'Brien. The O'Brien replicant had all of the real O'Brien's memories and in fact believed himself to be Miles O'Brien, most likely to make him more convincing. However, it was believed that a trigger would activate programming to assassinate someone at the upcoming peace talks on Deep Space 9 between the government and rebel forces, possibly the entire rebel delegation.
However, the rebels learned of this and warned Deep Space 9 personnel before the replicant arrived. The O'Brien replicant was understandably puzzled when his crewmates and wife began acting strangely around him. He became more suspicious when he was removed from working on security arrangements for the peace talks, and further more when logs showed his movements were being watched.
When confronted by station personnel, he fled the station in the runabout Rio Grande. He attempted to contact Admiral Rollman and Starbase 401 to inform her of the situation, but when she ordered him back to the station, he became convinced that whatever was wrong on Deep Space 9 was beginning to spread to all of Starfleet.
The O'Brien replicant then altered course towards the Parada system in the Gamma Quadrant with his crewmates in pursuit in the runabout Mekong. The O'Brien replicant managed to outmaneuver them and later followed the other runabout to Parada II where he discovered Commander Benjamin Sisko and Major Kira Nerys meeting with Paradan rebels. Believing them to be conspiring with the rebels, the O'Brien replicant trained his phaser on them, but was shot by one of the rebels. Seconds later, the real O'Brien emerged from another room, having been rescued by the rebels.
Coutu, one of the rebels, wondered aloud why the replicant had returned. The real O'Brien speculated that he was trying to warn someone something was wrong on the station. Apparently, the government scientists had done too good a job in recreating O'Brien, as the replicant had the same commitment to his duty and even expressed love for his "wife" before dying. (DS9: "Whispers")
The decision to use the word "replicant" when referring to the fake O'Brien was motivated by the desire to use something other than "android", which was deemed to be too familiar with the character of Data, or the term "clone" which didn't seem to fit in this instance. Writer Paul Robert Coyle ultimately chose replicant as an homage to the 1982 Ridley Scott film Blade Runner and the fact that the term hadn't already been used in Star Trek. Coyle commented; "Obviously, this guy wasn't a clone or an android or a robot. So what's left? I used replicant and nobody objected." (Star Trek: Deep Space Nine Companion, p. 118)
Colm Meaney said of playing the replicant O'Brien; "In order not to give away what was going on to the audience, we tried to keep him exactly the same, even though it was a different O'Brien. I played it exactly the way I usually do. It was only the circumstances around him that were weird and gave you the feeling that something else was going on." (Star Trek: Deep Space Nine Companion, p. 117)
In their review of "Whispers", authors Mark Jones and Lance Parkin expressed the opinion that duplicate O'Brien came across as "a man of integrity and determination." (Beyond the Final Frontier, p. 198)