In 1984 what is the torture O'Brien is inflicting?
You name it, and most likely O'Brien has inflicted it upon someone. Winston is the unfortunate victim to various different techniques, a few being starvation, time deprivation, brutal beatings, attempted brain-washings, psychological degredation, threats, and at the end, total immersion in something that Winston hates: rats. Through all of this physical and psychological torture, O'Brien is drilling Party mantra and "logic" into Winston's brain, systematically unravelling any sense of truth and right that Winston previously had clung to. He completely rewires Winston's brain to think in the perfect way that the party needs him to think. To do this though, O'Brien has to strip him of any sense of self pride, and take away absolutely any sense of individuality and honor that Winston might have.
There are a couple last straws for Winston. The first is when he sees himself in the mirror after being tortured for who knows long. He sees a skeletal, horrifying figure that only slightly resembles him. This terrifies and disturbs him so deeply that he finally caves, and does and believes anything that O'Brien tells him to. Because he can't even recognize himself anymore, he just becomes who the Party wants him to be, in order to gain some sense of identity back. The last straw is when he is threatened with the rats, and he begs them to put Julia in his place. This betrayal of Julia, something that he previously thought could never be taken from him, broke him. He realized that they could control everything, take every last bit of privacy in his brain, and make him do the unthinkable. There was no use resisting.
The physical torture is bad, yes, but it is how they deindividualize and humiliate Winston that holds the true power in O'Brien's techniques. I hope that helped; good luck!