Book 3, Chapter 2

Winston is moved to his own cell, where O'Brien can personally overview the torture. He first starts by interrogating Winston by attacking him every way imaginable. O'Brien asks questions for all the crimes Winston did and did not commit, but Winston had to agree to committing them or he would face endless pain.
Winston tried to hold up as long as he can and he notices that there are members of the inner party watching him. After trying to resist the torture in order to prevent himself from betraying Julia, O'Brien begin torturing Winston himself.
O'Brien attaches Winston to a machine with a dial that will send pain to Winston. O'Brien tests the machine to show Winston the capability of pain, he moves the dial to 40 which produces the feeling of having your spine snapped in half from the inside. He tells Winston that the machine can reach 100 - inducing a feeling similar to death. O'Brien asks questions to Winston and if Winston answers against the Party a surge of pain will attack Winston. Soon Winston accepts that 2 + 2 = 5 and that Oceania was always at war with Eastasia. Winston believes anything O'Brien tells him to avoid the pain, Winston also convinces that he loves O'Brien because he is the one who can shut of the machine and give Winston moments of relief.
O'Brien tells that Ingsoc, the Party, and Big Brother have perfected the ideas of Nazism and the Soviets. He tells that the Party executes any defiance without producing resistance and objection like during the Inquisition. He says that their methods force anyone to accept the views of Ingsoc as they cruelly believe it. They can force anyone to love Big Brother. They can erase anything or anyone from the fabric of time and rewrite the laws of nature as they please for "those who control the past, control the future and those who control the present control the past".
Winston accepts the ideas of Doublethink, and starts to believe his own memories are false.
O'Brien allows Winston to ask a few questions. Winston asks about Julia and O'Brien states that Julia had betrayed him almost immediately, a "textbook" conversion. Winston asks if Big Brother actually exists like he does; O'Brien states Winston no longer exists within these walls but Big Brother exists as a collective hive-mind and that it will always exist and can never die. Winston asks about the Brotherhood and if it actually exists; O'Brien tells that he will never know if the Brotherhood will ever exist. Winston asks about Room 101 and O'Brien states that everyone knows what will await them in room 101.