O'BRIEN: ‘There is a Party slogan dealing with the control of the past,’ he said. ‘Repeat it, if you please.’
WINSTON: ‘Who controls the past controls the future: who controls the present controls the past,’ repeated Winston obediently.

We saw the party maxim “Who controls the past controls the future: who controls the present controls the past” earlier, in Book One, Chapter 3, and its meaning is discussed at length in Famous Quotes Explained (the second quote). It appears again here in Book Three, Chapter 2 when O’Brien assumes a pedantic manner as he takes on the task of re-educating Winston (“More than ever he had the air of a teacher taking pains with a wayward but promising child.”).

‘But I tell you, Winston, that reality is not external. Reality exists in the human mind, and nowhere else. Not in the individual mind, which can make mistakes, and in any case soon perishes: only in the mind of the Party, which is collective and immortal. Whatever the Party holds to be the truth, is truth. It is impossible to see reality except by looking through the eyes of the Party.’

O’Brien continues in his teacherly role in Book Three, Chapter 2 as he patiently tries to get Winston to accept that the only “truth” that matters is what the Party says is the truth. You can read more about this quote in Quotes by Character: O’Brien (the third quote).

‘The first thing for you to understand is that in this place there are no martyrdoms.’

In this next excerpt from O’Brien’s intense grilling of Winston in Book Three, Chapter 2, O’Brien informs Winston that the Party isn’t going to kill him—at least until after they have successfully re-educated him, lest they create some sort of symbol for the resistance. You can read more about this quote in Quotes by Character: O’Brien (the fourth quote).

‘We do not destroy the heretic because he resists us: so long as he resists us we never destroy him. We convert him, we capture his inner mind, we reshape him.’

Here, in another quote from O’Brien during his interrogation of Winston in Book Three, Chapter 2, he explicitly states what the evidence up to this point has been suggesting, which is that the Party demands total control over the minds of its citizens and will not stop until this has been achieved. You can read more about this quote in Quotes by Theme: The Impact of Totalitarianism (the sixth quote).

‘We do not destroy the heretic because he resists us: so long as he resists us we never destroy him. We convert him, we capture his inner mind, we reshape him.’

O’Brien explicitly states in this quote from Book Three, Chapter 2 what the evidence up to this point has been suggesting, which is that the Party demands total control over the minds of its citizens and will not stop until this has been achieved. You can read more about this quote in Quotes by Theme: The Impact of Totalitarianism (the sixth quote).

‘Never again will you be capable of love, or friendship, or joy of living, or laughter, or curiosity, or courage, or integrity. You will be hollow. We shall squeeze you empty, and then we shall fill you with ourselves.’

In this quote from O’Brien in Book Three, Chapter 2, he, in effect, tells Winston what lies in store for him. Read more about this quote in Quotes by Character: O’Brien (the fifth quote).