Oblivion is the realm of the Daedric Prince Mehrunes Dagon, no mortal being ends up there in death.
Actually, Oblivion is the home of the Daedra and within Oblivion, there are several realms, not just Mehrunes Dagon's. The Shivering Isles, for example, is(is, are?) also a realm of Oblivion. All the more famous Daedric Princes(those that give the player a Daedric artifact in TES games) have their own realms of Oblivion.
"Durrr we go do aethurus and be wit the nine divines and all the bad peopelz go to Oblivian"
That's the impression I got.
What exactly gave you that impression? I didn't see any souls of deceased mortals in Mehrunes Dagon's realm of Oblivion and the few souls of deceased mortals in the Shivering Isles were those on the hill of suicides. The only reason those souls were on the hill of suicides is because they commited suicide in Sheogorath's realm, not in Mundus or in any other realm. It wasn't implied by the people of Cyrodiil and nothing in the game Oblivion supports what you claim, and especially not in the way you phrase it. Where are you getting these facts from? The people of Cyrodiil don't believe mortals go to Oblivion. They believe they go to Aetherius, even though they don't(but so do other other cultures). However, where are you getting the Oblivion part from and why do you phrase your claim like that?