Instructions
Clear the "Oblivion" cache only. Insert the "Oblivion" disc in your Xbox, then restart the machine. As "Oblivion" boots, hold down the "A" button on your controller until you see the Bethesda Softworks logo.
Play "Oblivion" for a while to see whether the cache problems have been solved. If you are still experiencing slowdowns, continue to the next step.
Select the "System" blade on the Xbox menu, then select "Memory." Highlight "HD," then press "Y."
Press the following buttons on your controller in order: "X," "X," "Left bumper," "Right bumper," "X," "X." You'll be prompted to perform "System maintenance" on your Xbox; agree to clear the machine's entire cache.