Ok, so you have a Pentium M. With the Pentium M and Core Duo series, you cant really go by they Ghz requirement, since they mainly refer to Pentium 4's which are clocked higher. Your CPU will be fine for running Oblivion, and it should run at 2Ghz during the whole game.
The only thing that may be preventing you from playing is your video card, is yours integrated or dedicated?