In 1984, when does Winston first realize that O’Brien is directing his torture?
By the end of Part II in the novel Winston's worst fears have been realized. His arrest which he had always believed to be inevitable has finally happened. He is seperated from Julia, and the start of Part III sees him imprisoned at the Ministry of Love in a group cell. There he meets others whom he had known at his workplace, and also witnesses a prisoner's terror at the prospect of being taken to to the dreaded 'Room 101'. At this time (the start of his imprisonment) Winston still rather naively harbours the hope that the Brotherhood can somehow smuggle him a razor so that he can commit a noble suicide. This hope is dashed with the entry of O'Brien.
Winston is stunned when O'Brien enters the room. Initially he thinks that O'Brien has also been arrested, but in an ironic twist O'Brien said that they had 'got' him a long time ago. O'Brien is in fact a highly loyal Party apparatchik, and he is set to subject Winston to some very effective torture and brainwashing. O'Brien coolly brushes aside Winston's shock at seeing his true identity,
"You knew this, Winston," said O'Brien. "Don't deceive yourself. You did know it - you have always known it."
Yes, he saw now, he had always known it. (p.251)
Winston's fantasies of the Brotherhood as led by O'Brien somehow creating a just and truthful society of the future have suddenly come crashing down. He is forced to realize that his fascination with O'Brien had blinded him to the dangers of O'Brien's duplicity. It seems like another resignation to fate - a major flaw in Winston's character, and it says much about their relationship that even as Winston is later being cruelly tortured and 'cured of his insanity' he still seems to retain a morbid fascination for the charismatic O'Brien.