Was Oblivion really buggy on day one? I know Morrowind and Daggerfall were, but didn't hear much about Oblivion.
Oblivion launched in an OK state but Fallout 3 and New Vegas didn't.
Gamebryo alone constitutes as a bug. I had a horrible stuttering problem, which wasn't caused by low FPS, but a desync between the graphic card, the game engine and the monitor. Game ran the graphic card with 64Hz, while the monitor could do only 60Hz, therefore every 60 frames 4 were skipped. I had to do some magic tricks with the .ini to at least limit this to a different, but barely noticeable problem (if the game want under 60 FPS, slowdowns occur).
I have the same issue with all the other Gamebryo games that means both new Fallouts.
Other than that, the game had a nice nest of game braking, quest destroying, audio missing, world disfiguring, crash inducing, items missing, effects corrupting, balance disrupting and other minor nuisance game bugs. I think the last Unofficial (read: better) patch made somewhere around 2500 fixes as a whole.