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. The Party therefore suppresses intellectualism in favor of ignorance, which it promotes as strength.
The torture scenes undertaken by O'Brien were influenced in part by the stories leaked out of the USSR of the punishments inflicted on political prisoners in mental hospitals and the Gulag. Winston suspects that O'Brien secretly opposes the Party.
O'Brien invites Winston who then invites Julia to his flat where, as a member of the Inner Party he lives in comparative luxury. From the very beginning it becomes apparent that Winston is a curious person.
Other than a desire for intercourse and their hatred for the party, the two have very little in common. However, as an Inner Party member, he has access to far better cigarettes and other goods than they do.
He is not a sheep that believes everything that is thrown at him. Written inthe novel is set in a futuristic totalitarian state referred to as Oceania.
The character of O'Brien is not so different from many of the contemporary leaders of the 20th century.