What? No. Oblivion is as well defined as can be. It's the void that the Daedric and Aedric realms/plane(t)s reside in. It's beyond the Mundus/Creation, and beyond Oblivion in turn lies Aetherius. If the planes/realms are eggs floating in water, then Oblivion is the water. Except it's an unending expanse of nothing instead of liquid.
While probably not an authoritative source, the info in the TES wikia say this about Oblivion:
"Oblivion, also called The Void of Oblivion or the Planes of Oblivion and also Wasts of Oblivion, is a collective term used to describe one subset of the planes of existence. The planes of Oblivion are those which have the least connection with the mortal plane, Nirn.
The planes which make up Oblivion are defined primarily by exclusion. Any plane which is not Nirn itself, not one of the planets or moons, and not Aetherius itself, is generally considered to be an Oblivion plane. It is commonly believed that there are an infinite number of such planes, though Tamrielic scholars are aware of only a small handful. In particular, it is believed that each of the sixteen Daedric Princes rules over their own plane, which is formed into an image of their choosing."
While some of the books in game call Oblivion "a place", they also refer to the planes as "lands" and that it is "composed" of them. Nowhere is Oblivion, to my knowledge, acknowledged as an actual separate entity, or does someone travel "to" Oblivion, only as the aggregation of the different planes. And not even all Planes are part of Oblivion. Roughly, they are defined as "planes run by the Daedra Princes" or "Planes that are not Nirn".
I'm going by in-game Lore, by the way. Don't know much about novels.