In 1984 what does O'Brien know about Winston that allows him to break Winston's resistance?
Unfortunately, O'Brien seems to know absolutely everything about Winston, and any possible defenses and thoughts he is having. Even in the beginning phases of confinement and torture, O'Brien often interrupted Winston's thoughts and say, "I know you're thinking....." and then he would go on to describe Winston's very thoughts. It's rather eerie, and alludes to the possibility that O'Brien has been doing this for a very long time, and torturing many, many people, so he knows their thoughts. Also, it hints that they had their eye on Winston for a long time, and were watching his actions and the way he lived and thought. During the torture itself, Winston tries clinging to his memories of Julia, and his memories of his mother and sister to stay sane. In the end though, even that doesn't work.
The two pieces of information that O'Brien uses to finally break Winston are what Winston fears most and loves most. Winston fears, above all, rats. And he loves, above all, Julia. So, to get Winston to betray Julia, he threatens Winstons with rats. The rats terrify Winston so much that he will do anything to get out of being tortured by them, even if it means turning that torture on Julia. And, that is what he begs O'Brien to do--torture Julia with them instead. This act betrays his love for Julia. He realizes that he had been naive about thinking they could never touch his love for her. They could and did, and he was willing to sacrifice her to save his own skin. That is the last straw, and Winston is there. O'Brien took two things at the opposite polar extremes--most loved and most feared--to break him down.