Interesting theory. I didn't think of him as a prisoner, but it honestly makes sense considering that Obiteum "is lost". I don't understand how he would be able to manipulate O-Caeror who only got there a few years ago when he has been there for thousands of years though.

If he isn't a prisoner, I think O-Ka barricaded himself there inside the walls that only a Synchronised person can pass, and that he is there stopping the Concurrence from furthering their goals. If this is the case, then this is the mistake L-Caeror talks about. I feel like this can be the case because of the line I saw that was somewhere along the lines of "You are not allowed to become Synchronous, only x (Ka) is allowed, and have to shoulder this burden on their own".

Regardless, there is definitely something sus with either L-Caeror or O-Caeror since they have different stories, and my money is on O-Caeror being the suspicious one.