This is a common problem with Oblivion... but thankfully it is one that is easily fixed.
To solve this problem, reset your Xbox, and hold down the A button as the game is booting up. If any button is held while Oblivion is loading, it will clear its hard drive cache, and create a new one.
Once you see the “Bethesda Softworks” logo video, the game is already loaded, and the cache should have been cleared.
Note, this does not effect your game in any way other than loading. You will not lose any saved data.
Also check to make sure the disc surface is free of dust and smudges, even a light smudge can cause the game to load slowly or lock up completely.
If the above doesn't work try removing your hard drive completely.
If that doesn't work, or you don't have a hard drive, you're experiencing a completely different problem and should probably contact MS.This post has been edited by twistedsymphony: May 13 2006, 07:35 PM