When Winston and Julia finally get to speak to O'Brien in his rooms, he pretends to be a key leader in the Brotherhood, the rebel movement that champions Goldenstein and is working covertly to bring down Big Brother. As such, O'Brien asks Winston and Julia what they would be prepared to do to become rebels. This of course is used against them later on. He appears to have mastered successfully what Julia and Winston try to master: a double life, where he can appear to be a loyal citizen of Big Brother and an important worker of the party on the one hand, but internally, a rebel.
However, it is only when the Thought Police arrest Winston and Julia that he sees O'Brien in the Ministry of Love and realises how he has been tricked. O'Brien seems to be a strange mixture of torturer and father-figure as he watches over Winston's reconditioning to love Big Brother, and it seems that he, above all, symbolises the tyranny and the autocracy of Big Brother's might and eternal power.