In the totalitarian, dystopian society depicted in George Orwell's 1984, the possibility that those with whom one comes into contact may not be what they seem is a constant risk. Such is the case with the character O'Brien. This mercurial figure crosses paths with Orwell's protagonist, Winston, only fleetingly during the years that precede the beginning of 1984, and Winston is uncertain what, exactly, to make of O'Brien. A large, burley man, O'Brien is also refined in a way that contrasts sharply with his public appearance. Early in Orwell's story, the novel's narrator makes the following observation regarding Winston's relationship to this seemingly authoritative figure who may be less doctrinaire in his fealty to Party dictates than his official position demands:
"Winston had seen O'Brien perhaps a dozen times in almost as many years. He felt deeply drawn to him, and not solely because he was intrigued by the contrast between O'Brien's urbane manner and his prize-fighter's physique. Much more it was because of a secretly held belief--or perhaps not even a belief, merely a hope--that O'Brien's political orthodoxy was not perfect. Something in his face suggested it irresistibly."
That "secretly-held belief" constitutes the crux of the matter, and to fully appreciate the importance of O'Brien in Orwell's society, it helps to recall a real-life operation carried out by the newly installed Bolshevik regime that subverted a legitimate revolution against a czar only to impose an even more draconian dictatorship on the very people it had presumed to help.
Immediately upon taking control of the major seats of Russian power, the Bolsheviks understood the need to protect themselves against the machinations of internal and external enemies alike. The czar's secret police, then, was replaced with the Bolshevik's version of a repressive, omniscient force, the Cheka (an acronym for the Russian words for Extraordinary Commission for Combatting Counter-Revolution and Speculation). The Cheka's leader, Felix Dzerzhinski, ordered the establishment of a fake anti-Bolshevik resistance group operating in Western European capitals. This fake organization was called "The Trust," and it successfully attracted Russian emigres eager to assemble and conspire against the communist revolutionary movement systematically entrenching itself in Moscow and St. Petersburg. The Trust operation was, in fact, successful at provoking "disloyal" Russians into identifying themselves, making their eventual capture much easier than would otherwise have been the case.
The reason for this bit of Russian history is because Orwell modeled his autocratic society after the Communist Party of the Soviet Union, and the O'Brien character's references to the Brotherhood serve as metaphor for the real-life Trust operation. In discussing the figure of Emmanuel Goldstein and his "vast shadowy army, an underground network of conspirators dedicated to overthrowing the State," Orwell's narrator, and O'Brien, allow for the possibility that (A) the Brotherhood exists and threatens the Party, and (B) the Brotherhood doesn't exist other than as an instrument of the Party's propaganda and a means of furthering its repression. As Winston's commitment to the Party begins to weaken, and the image of the Brotherhood holds out hope of an alternative, the seeds are planted for the protagonist's newly conceptualized concerns about the Party's legitimacy to develop into a full-fledged position of resistance. Winston now risks identifying himself with subversive elements to which O'Brien has introduced him. If, as seems likely, the Brotherhood was modeled after the Trust, then it will have served its purpose of deceiving potential opposition movements into revealing themselves and, consequently, making themselves vulnerable to isolation, capture, and destruction.
O'Brien's betrayal succeeds because of his manipulation of others into revealing themselves as possible enemies of the state and, so doing, setting themselves up for destruction.
O'Brien pretends to be part of a secret underground movement working against the government in order to lure Julia and Winston into confessing their mutual desire to overthrow the state. They even agree they would do so violently. But O'Brien is not merely an agent working to arrest those who commit thought-crimes. He is a true believer in the system for which he works. He believes that power--which he defines as the boot stamping the human face--is the ultimate arbiter of truth--in fact, that power IS truth. He believes truth is whatever the people in power say it is and not an objective reality. He not only forces Winston to assent to this idea by agreeing that two plus two is five but to actually come to believe two plus two is five. He also breaks Winston's spirit by playing on his deepest fear in order to get him to betray Julia. O'Brien doesn't just imprison and torture Winston, he robs him of his humanity: both his free will and his capacity to love another being. At the end of the novel, Winston is simply an empty shell, outwardly human, inwardly broken.
O'brien is a government agent who poses as a member of The Brotherhood. He offers Winston the collective wisdom of the resistance for the sole purpose of entrapping him and his partner Julia.