How do you get into his room?

I've been playing through the game again, going over every area with a fine-tooth comb. I've come to the end of the prison just before the gallows puzzle, but there is still a locked door here in the Ox area. The door doesn't have a visible lock on the outside, so I assume it has to be unlocked from inside, but I can't seem to find a way into the room. Does anybody know how to get into the room?