O'Brien, with whom Winston feels an unaccountable kinship from the first, seeks him out one day to talk to him about an article Winston recently wrote (Part I, chapter 6). O'Brien says he's read the article and noted that Winston used a couple of words that are now obsolete, and invites Winston to come to his house sometime to have a look at an updated dictionary of Newspeak, which Winston doesn't yet have. Winston realizes that O'Brien is using this subject as a pretext for passing on his address. He wants Winston to visit him.
Winston has always had a strange feeling about O'Brien, that he secretly hates the Party as much as Winston himself does, and speculates that he might be part of an underground rebel movement known only as 'The Brotherhood'. Winston thinks maybe O'Brien is covertly inviting him to join The Brotherhood. O'Brien is not to be trusted however - although this is true of practically everyone around Winston, as the atmosphere in which Party members live is always one of suspicion and mutual distrust. In fact, O'Brien is the most loyal of Party members and is really luring Winston to his destruction by pretending to be against the Party. When Winston is finally captured, it is O'Brien who acts as his chief torturer and attempts to brainwash him into accepting the Party and all its brutal, malign, and fundamentally dishonest ways.
The interesting thing is that, although Winston is tricked by O'Brien's ruse, on another level it hardly seems to matter to him whether O'Brien is a friend or an enemy. The main point is that O'Brien seems to always understand him, and even during the long hours of agony, both physical and mental, inflicted on him by O'Brien in the Ministry of Love, he never loses this sense of rapport with him:
The peculiar reverence for O'Brien, which nothing seemed able to destroy, flooded Winston's heart again. How intelligent, he thought, how intelligent! Never did O'Brien fail to understand what was said to him. (Part 3, chapter 3)