Oblivion is the name given by the Lothanasi to whatever lies on the other side of the Veil. It is not a place, per se, so much as an idea, and a hazy one at that. It is considered to be part of the Realms Beyond.
Theists believe that Oblivion is the realm of the Creator; some even call it "the True Heavens" or "the Far Heavens". Some Universalists believe that it is a realm of pure thought (or pure Numen, where the scattered fragments of the Great Maker are slowly reassembling themselves. Others think that Oblivion is a cosmic recycling center, where souls are returned back to the material world to be reborn in new bodies.
The truth is that nobody really knows what Oblivion is, and what happens to the spirits who go there. All we really know is that, once a soul passes beyond the Veil, it never comes back.
Well … except, possibly, for one.