Winston does not even realize that he has been "betrayed" by O'Brien until after he is imprisoned. When he encounters O'Brien in the Ministry of Love, he first exclaims, "They've got you too!" O'Brien tells him, though, that he has always known that O'Brien was with the Party. Still, his imprisonment means that his resistance to the Party has always been illusory. His belief that there was a "Brotherhood" full of sympathetic figures who would fight for liberation from Party control was misguided.
With his arrest, he is imprisoned and tortured physically and psychologically by O'Brien until he loves the Party. The incident illustrates the futility of resistance to the Party because of its thorough control over every aspect of life. The incident also allows Orwell to explore the dynamics of totalitarian rule. O'Brien discusses in great detail how the Party successfully manipulates the minds of Party members, distorting reality to the point where the Party itself is the only source of truth. O'Brien also makes clear the end to which the Party exercises its rule. This is a question that Winston posed early in the book—the "why." O'Brien says that power itself is the aim of the totalitarian state, of Big Brother. O'Brien betrays Winston, but in doing so, he reveals the source of totalitarian power.