Oopsie is a clown.
He was sitting with the other clowns in Krusty's car and had to flee from the car after Bart stole a tank and went to run over the car with it.
When Dwight took hostages at the First Bank of Springfield he told them that he will let them live as long as there's no funny stuff. Krusty told him that he didn't need to worry about him as he was voted America's least funny clown in Laff Ways. He was therefore worse than Scuzzo, Scummo, Oopsie, Carlos Mencia, Stinko, Blumpy and Sergeant Serious. Krusty told him that he didn't like it as he didn't understand how he could be worse than Sergeant Serious, for he has stolen all his jokes.