Felix the Grouch is an old friend who visits Oscar in a 2000 episode of Sesame Street. Oscar is disappointed to learn that his friend Felix has become a neat freak, and he worries that he isn't a real Grouch anymore. However, they argue so much that Oscar realizes that Felix is just as grouchy as ever.
The character of Felix is a spoof of the 1965 Broadway play The Odd Couple, written by Neil Simon. In the play, as well as the subsequent movie and TV sitcom adaptations, two divorced men share a New York City apartment -- the messy Oscar Madison (originated by Walter Matthau) and the neat Felix Unger (originated by Art Carney and later played by Jack Lemmon).