O'Brien is a prominent leader in the Inner Party, although his official title is not clear. He seems to be close to Big Brother and may even be part of a collective that makes up Big Brother. O'Brien seems to be a co-conspirator and friend to Winston Smith until the third part of the novel, when he is revealed as a zealous Party leader who had been closely watching Winston for years.
O'Brien represents the Party and all of its contradictions and cruelty. He functions largely to bring the reader into the inner chambers of the Party so that its mechanisms can be revealed. Without O'Brien, the Party would be as mysterious to the reader as it is to Winston and Julia.
While Winston is characterized as an individual, a small man in a large society, O'Brien is bigger than life and remains so throughout the novel. This effect is partly a result of his mysteriousness and partly because the novel hinges on O'Brien's "turnabout" actions; if he were given more time on the page, his true nature would have been revealed too soon.
O'Brien is not only duplicitous in nature, but he also seems to be able to employ doublethink very well. Whether or not he truly believes contradictory notions simultaneously, he is determined to teach Winston to do so. There is no evidence to sustain the idea that O'Brien truly believes in the concepts that he forces upon Winston beyond his statement to Winston in the Ministry of Love that the Party had gotten him (O'Brien) long ago.
This statement illustrates a consciousness that would be dangerous for an Outer Party member to have, so it is possible that O'Brien shares the same consciousness as Winston, but because of his status in the Party, has no reason to want society to change. He is not the individual being tortured, though he would have Winston and the reader believe that the "rehabilitation" once happened to him as well.
O'Brien is often seen as a father figure and a friend to Winston. O'Brien is trying, through torture, to make Winston "perfect," to "save" him. If Winston would simply embrace the Party's doctrine, he would be "clean." But it is not really Winston that O'Brien and the Party want to change; the Party wants to purify all thought, believing that one stray thought has the potential to corrupt the Party.
The character of O'Brien is not so different from many of the contemporary leaders of the 20th century. For example, Hitler and Stalin used this kind of torture to keep their power and did it in the name of "purity." O'Brien represents these leaders and others, who use cruelty and torture as their primary method of control.