What was the truth about O'Brien ?Was he really a member of the Brotherhood or was it just an act that he put to trap Winston?
O'Brien is kind of a jerk. Your subtext correctly identifies that he was not a collaborator with Goldstein nor a member of the Brotherhood, but rather a covert agent for the government. His desire and motivation is to entrap the Winstons of Oceania and turn them over to Big Brother for a type of reeducation. O'Brien has decided to act completely in the interests of the state and channels his intellect to this end. Once he captures Winston, O'Brien helps to reorient him away from "thoughtcrime" and to a realm where complete and open love for Big Brother is evident. Through the use of Winston's darkest fear of rats, O'Brien is able to get Winston to renounce everything, until he is nothing more than a gutted shell that becomes a vessel for the government, only to be eliminated at that point. When asking about "the truth" for O'Brien, I think that the only truth that is present for him is the consolidation and strengthening of the rule of Big Brother. There does not seem to be any other truth under which he operates.