My game is slowing down during gameplay, how can I fix this?
Oblivion uses the Xbox 360 hard drive extensively to cache (copy and reuse) game data. This is done to optimize all loading the game does. It has been found in rare cases on some Xbox 360s that the cached data can get overly fragmented. This exhibits itself by the game taking a long time to load anything from menus, dialogue, sounds, objects, levels, and more.
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.
If you have a wirless controller the best way to ensure it has been cleared correctly is to switch the Xbox 360 on by pressing the Eject button on the front of the console. This will start the 360 up as far as the Dashboard. Once at the dashboard connect the wireless controller by holding the Xbox Guide button (silver/green X in the middle of the controller).
Now that the controller is connected move down to the Close Tray option and press A, immediately press A again and keep it held until the Oblivion logo appears. Now let go of A, press Start and select Continue to reload the game.