Early in the novel, O'Brien is developed primarily as the subject of Winston's fantasies. Despite only having seen the man a few times, Winston imagines that O'Brien, a Inner Party official, is in reality a Party discontent like him:
He felt deeply drawn to him...because of a secretly held belief — or perhaps not even a belief, merely a hope — that O’Brien’s political orthodoxy was not perfect.
This conviction only increases over time, and O'Brien eventually approaches him, leading him to believe that he is, in fact, a Party dissident. Winston and Julia go to his home, where they confess their heretical thoughts, and throw themselves on his mercy while asking if the rumors about the Brotherhood are true. O'Brien engages in what turns out to be an extended act of deception, convincing Winston that he is to be made part of the resistance. He even drinks to the health of the Party's bugaboo Emmanuel Goldstein:
I think it is fitting that we should begin by drinking a health. To our Leader: To Emmanuel Goldstein.
This quote, in retrospect, reveals the extent of O'Brien's duplicity and ruthlessness. Later, when Winston is arrested and brought in for torture (by O'Brien himself) we see another quote that reveals the true nature of the Party and of O'Brien. After several lengthy torture sessions in which O'Brien reveals the true nature of the Party and of Big Brother, he offers a chilling vision of the future:
There will be no curiosity, no enjoyment of the process of life. All competing pleasures will be destroyed. But always — do not forget this, Winston — always there will be the intoxication of power, constantly increasing and constantly growing subtler. Always, 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 — for ever.
This passage succinctly characterizes O'Brien. Like the Inner Party that he represents, he wields power for its own ends. He is highly attuned to the processes by which memory is manipulated, created, and destroyed. He is ruthless, cruel, and totally bereft of any sense of humanity. He embodies the frightening methods and logic of totalitarianism.