The baseball sequence was filmed at Shea Stadium before a regularly scheduled contest between the New York Mets and Pittsburgh Pirates on June 27, 1967. Originally, Roberto Clemente was supposed to hit into the triple play. When he refused, Bill Mazeroski took his place.
The names of the English sisters, Cecily and Gwendolyn, are taken from Oscar Wilde's play "The Importance of Being Earnest".
Based on the experiences of Neil Simon's brother when he got divorced.
Billy Wilder was going to direct and write the screenplay at one point.
Despite the fact that a number of key scenes involve poker games, and the fact that the five male lead characters are all poker buddies, Felix is never seen actually playing poker.
Walter Matthau, who played Oscar in both the original Broadway play and the movie, actually asked the play's author, Neil Simon, if he could play Felix instead. This was because Matthau thought Oscar's personality was too similar to his own so the role would be too easy, whereas playing the persnickety Felix would be a real acting challenge. Simon replied, "Walter, go and be an actor in somebody else's play. Please be Oscar in mine." and Matthau finally agreed.
Roberto Clemente was asked and accepted the role to hit into the triple play at Shea Stadium. However, the fleet footed Pirate kept beating the throw to first base. After several takes, Clemente slowed so much he appeared to be walking. Bill Mazeroski, a more leaded footed athlete, was offered the part instead.
Neil Simon based the character of Felix on his older brother, Danny.
The play starred Walter Matthau as Oscar, and Art Carney playing Felix. When they were making it into a movie, they felt Carney didn't have enough box office punch, so they cast Jack Lemmon instead.
The original Broadway production of "The Odd Couple" by Neil Simon opened at the Plymouth Theater on March 10, 1965, ran for 966 performances was nominated for the 1965 Tony Award for the Best Play.
During one scene, a sports radio program playing in the background reports an item about a baseball trade involving a player named Hank Moonjean. Hank Moonjean was actually the assistant director on this film.
Cecily (Monica Evans) says that her sister, Gwendolyn (Carole Shelley), watches over her "like a mother hen." Both Monica Evans and Carole Shelley also have speaking parts in Disney's Robin Hood (1973) in which Shelley plays a hen (Lady Cluck) and is a companion to Maid Marian (Evans).
The second pairing out of ten movies of comic actors and great friends Jack Lemmon and Walter Matthau.
Walter Matthau won the 1965 Tony Award (New York City) for Actor in a Drama for "The Odd Couple" as Oscar Madison and recreated the role in the movie version and it's sequel.
At one point in the story (set in New York) while trying to encourage Felix to go out on the town with him, Oscar says "Getting a clear picture on Channel 2" is not his idea of a fun evening. For years though the mid to late 70s and early 80s this movie was a staple of WCBS Channel 2's "Late Show" movie in New York City.