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 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.
