While Winston is being held prisoner, O'Brien walks in the room for the first time and the conversation goes something like this:

Winston: They got you too!

O'Brien: Yes, they got me a long time ago.

Does this mean that O'Brien, like Winston, was once a rebel, but was caught and converted? If so, how come O'Brien hasn't been shot already? We learn in the book that even if people are reformed, they still get killed.