The mirror-universe Miles O'Brien, nicknamed Smiley, is a Terran slave and tinkerer on Terok Nor who joins the rebellion and later becomes one of its most dependable leaders.
In the mirror universe Miles O'Brien is a downtrodden Terran labourer who keeps the machinery of Terok Nor's ore processing centre running. His counterpart's captain, the mirror Sisko, gave him the mocking nickname Smiley.
In Crossover he helps Julian Bashir escape from ore processing after hearing that in another universe a man like him is a respected chief of operations. He asks to be taken along, and ends up joining Sisko's newly declared revolt.
In Through the Looking Glass (2371) Smiley crosses to the prime universe and abducts Benjamin Sisko at phaser point so that he can pose as the dead rebel leader. He returns in Shattered Mirror, where he has stolen the plans for the Defiant, and in The Emperor's New Cloak.