Castle..um...what was it again?
This is for all those out there (You know who you are) who can't remember where Organization XIII's main base is.
The-Castle-That-Never-Was (You know, on The-World-That-Never-Was) is the Organization's main base.
Castle Oblivion is more of a secondary base. It is NOT located on The-World-That-Never-Was.
In case you've forgotten that; it is not, never-even has been and never will be on The-World-That-Never-Was.
http://en.wikipedia.org/wiki/List_of_locations_in_the_Kingdom_Hearts_series
Followed the link? See? Two seperate locations!
*Breathes deeply* Right...rant over!
Edit: Apparently, according to the "Secret Ansem's reports", the Castle Oblivion is/was found on a place called "The World-in-between". That is not its actual name, as far as I can gather.