If you want to uninstall it I think you'll have to do it manually. Hurray for DRM!
In order to do this you'll have to do delete files from three locations; the install path, the game data path and the registry.
Before you begin I suggest taking a backup of at least the registry since messing around with that can cause problems especially if you don't know what you are doing.
The Install path
This is where you installed the game to in the first place. Generally it will be
C:\Program Files (x86)\Bethesda Softworks\. Delete the Oblivion folder. If you chose a custom location you'll have to find it. You can easily find it by right clicking the shortcut on the desktop or from wherever you launch the game, and select
open file location.
The Game Data
This is where the game automatically saves your save files and other things such as user preferences. If you wish to delete these go to
C:\Users\[insert PC name here]\My Documents\My Games\ and delete the
Oblivion folder. If you think you may play the game later on you may want to leave this folder alone and it will be available the next time you install the game.
The registry entries won't take up much space so you may wish to just leave it but if you wish you could try either doing it manually or using some tool. To do it manually: open up the registry by pushing winkey + r and typing
regedit, hit enter and the Registry Editor should open up.
Just search through the registry using the Edit > Find option for keywords such as
Oblivion and then delete all entries.
A tool such as CCleaner searches through the registry and deletes unused registry keys. The Oblivion entries should be unused at this stage and thus be deleted when you run CCleaner.