Overview
Winston suspects that O'Brien is secretly opposing the Party. Eventually O'Brien approaches Winston with some leading remarks which seem to confirm Winston's suspicions. Winston finds the courage to approach him candidly, declaring himself an enemy of the totalitarian state. At first, Winston's intuition seems to be correct: O'Brien presents himself as a member of the "Brotherhood" seeking to overthrow the Party.
When Winston is later arrested, it turns out that O'Brien is an agent of the Thought Police, and is completely loyal to the Party. He reveals himself as he enters the cell by responding to Winston's exclamation ("They've got you too!") by wryly commenting, "They got me a long time ago."
O'Brien is part of a false flag resistance movement whose real goal is to find thought-criminals (citizens who think something that is deemed to be unacceptable by the party), lure them in by pretending to be on their side, then arrest and "cure" them. O'Brien tortures Winston to cure him of his "insanity," in particular his "false" notion that there exists an external, self-evident reality independent of the Party; O'Brien explains that reality is simply whatever the Party says it is.
He is entirely honest about the brutal cynicism of the Party; the Party does not seek power to do anything good, but simply to revel in that power: "Always, Winston, at every moment, there will be the thrill of victory, the sensation of trampling on an enemy who is helpless. If you want a picture of the future, imagine a boot stamping on a human face — forever."
Even in the torture scenes, there is a strange intimacy that persists between Winston and O'Brien, who displays an uncanny ability to infer what Winston is thinking. O'Brien even states that Winston's mind appeals to him, and that it resembles his own mind, except that Winston happens to be insane. Eventually, in Room 101, O'Brien tortures Winston into submission so that he "willingly" embraces the philosophy of the Party.
Read more about this topic: O'Brien (Nineteen Eighty-Four)