Wired did a feature on him several months back. The Olympic committee and other full-bodied contestants don't want him competing in the regular events because they feel his Cheetahs (that's what they're called) artificially lengthen his stride, which gives him an advantage.
The engineers don't agree, because Oscar doesn't have calf muscles that give that extra burst. The Cheetahs have a bit of springing action which makes up for it, but not completely (supposedly).