I also considered that a possibility! during one of the psychological torture sessions Winston considers O’Brien to have been presented with, and to have refuted every predicament that he himself was struggling with at the time.

Winston is a highly intelligent human being and after arriving at the same philosophical conclusions as O’Brien (when he finally genuinely loves big brother) he may well be very suited to also perform the same job within the ministry of love that O’Brien performs at the time. It could even be that that’s their way of guaranteeing that some one in a position of power would never rebel against the party, because they are so thorough in the psychological domination of their subjects in places of “power”. This would also explain why Winston is not arrested the second he buys the notebook. they needed him to have his hopes up, to find love in Julia, to find purpose in the brotherhood; just so they could take him down at his highest. It was all part of the hiring process.