When does Winston first realize that O'Brien is directing his torture?

In Part Three, Chapter One, amid the many beatings he has endured, O'Brien appears to Winston in his cell and reveals himself as the director of his torture. For Winston, O'Brien's true identity is a fact that, deep down, he had always known:

"Yes, he saw now, he had always known it."

At this particular time, however, Winston does not fully grasp O'Brien's role. This happens in the next chapter (Chapter Three) when he is taken to a bright room where O'Brien appears with a man in a "white coat." It is in this room that Winston makes this important realisation about O'Brien:

"All through his interrogation, although he had never seen him, he had had the feeling that O’Brien was at his elbow, just out of sight. It was O’Brien who was directing everything."

He realises that O'Brien is the master of his torture: that he controls when he is tortured, for how long and by what means. He will also decide if and when Winston is killed. But O'Brien is not interested in killing Winston, it is his plan to make him love Big Brother.