Which south park character and in which episode says Oh Noooo in a high pitched voice he sounds kind of like cartman?

The character you're referring to is Eric Cartman from the "South Park" episode titled "A Scause for Applause," which is Season 16, Episode 13. In this episode, Cartman exclaims "Oh nooo!" in a high-pitched voice during a scene that emphasizes his exaggerated reactions. This moment captures Cartman's typical blend of humor and melodrama.