What does O’Brien mean when he tells Winston in Book 3 of 1984, “They got me a long time ago”?

When O’Brien tells Winston in Book 3, Chapter 1, “They got me a long time ago,” he might be saying that he was once a dissident but has been turned into an orthodox Party member. However, this is unlikely, as dissidents are usually vaporized some time after they are tortured and released back into society. It is more likely that O’Brien is saying that he is a Party loyalist and that he has known for a long time that there is absolutely no escape from Big Brother. In fact, O’Brien says he helped write “the book” in order to figure out who is against the Party in order to eliminate them. In doing so he is serving as an effective instrument of the Party, helping to strengthen its power over the people.