The way I see it is that O'Brien is a standin for the party in 1984 and when he is being praised by Winston it's Winston praising the party.

It's the same way many people under totalitarian regimes sincerely think. Even when the state is torturing and murdering people.

Julia is a standin for escape from that system and freedom. Winston knows he wants it, but beyond very basic feelings can't articulate why.