Yes, it's the moment all you Oblivion PC owners have been waiting for. Someone has actually managed to get a working mod to allow co-op play on the PC version of Oblivion! The mod is still in early form, but you can keep up with it's progress at the link below!

Source: http://z3.invisionfree.com/MultiTES4/index.php?s=349eb3c32eb0cb868c5a9be21986b5f2&showtopic=84