There are conflicting parts of the book that keep me from making a solid conclusion about this, but First let us establish that Winston is also sane by our society's standards.
-When O'Brien is torturing Winston, he tells him that Winston's mind reminds him of his own, except that Winston is insane
- When he is interrogating Winston earlier, he seems very convinced, with an air of confidence, that everything he says about the party being omniscient and all of that is true. But Winston still clings to his own truly correct memories.
- But when Winston is in O'Brien's house, he seems completely aware about how evil and corrupt The Party is and how they need to be taken down.
So, even though he is lying, O'Brien denounces the party and is sane enough to come up with the thoughts, yet, based on Winston's words when he is tortured, O'Brien seems completely certain that the party is all-knowing and all-powerful. Which is not sane by our society's standards.