Ok heres my theory.
- I think that Oz is one of the keys that the Dukes have to their door. Here is the article about it. It says that the keys have no shape, so I think that Oz is the key of the Vessalius family or even of the Baskerville family (since Zai Vessalius, his father took him after his birth so it is possible)(he is a baskerville).
- There is the Will of the Abyss and there is also something else. I cant remember now but I know that there are two things that you can acquire to control the Abyss. So I think that Oz is the other thing that the Will of the Abyss wants it to join her. The Will of the Abyss has Alice as its vessel (the white Alice that I think is actually Lacie), and that wants the other thing that can control Abyss to join her.
I think its confusing to read my horrible description, so sorry. I found on one article that there are two things that you can acquire to control the Abyss but I was looking ages for it and can't find it. So what do you think?