According to former Paramount production chief Robert Evans in his memoir "The Kid Stays in the Picture", producer Howard Koch originally wanted to use the Broadway cast, Walter Matthau (Oscar) and Art Carney (Felix) in the movie. Evans wanted Jack Lemmon for Felix. Evans also wanted Billy Wilder, who directed Lemmon and Matthau in The Fortune Cookie (1966), as writer-director. The cost for the Lemmon-Matthau-Wilder package was $3 million plus 50% of the profits. Paramount owner Charlie Bluhdorn balked at the demands and personally took over negotiations. Wilder eventually dropped out. Lemmon was signed for $1 million against 10% of the gross and Matthau got a straight salary of $300,000.
Walter Matthau, who played Oscar in both the original Broadway play and the movie, 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 and 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." Matthau finally agreed to it.
While the names of the sisters - Cecily and Gwendolyn - are the same as the female leads in Oscar Wilde's play "The Importance of Being Earnest," Neil Simon claimed in interviews that it was unconscious and the coincidence didn't occur to him until years later.
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. 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 lead-footed athlete, was offered the part instead.
The movie set a record at the Radio City Music Hall during its initial engagement. Of the 640 films that had played the theater in its first 42 years, it was the leader in terms of run (14 weeks) and total gross (over $3 million.)
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.
The original Broadway production of "The Odd Couple" by Neil Simon opened at the Plymouth Theater on March 10, 1965, ran for 966 performances and was nominated for the 1965 Tony Award as Best Play. Walter Matthau won the 1965 Tony Award for Best Actor in a Play.
At one point in the story (set in New York), Oscar says "Getting a clear picture on Channel 2" is not his idea of a fun evening. For years through 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.
When Felix is upset over his impending divorce, Oscar offers him a box of Mallomars to make him feel better. Since Mallomars are only "in season" from September to March, it's unlikely that there were any in the apartment since the scene took place on a hot summer day. Of course, considering Oscar still has his Christmas decorations up in July, and serves "green" sandwiches that might be very fresh cheese, or very old meat, one can safely assume he keeps Mallomars for well past their best before date.
At about the 1 hour mark, Jack Lemmon enters a Bohack supermarket which was located at the SE corner of 87 St. and 2nd Ave. This is diagonally across from where "The Prisoner of Second Avenue"(1975) was filmed at 245 E. 87 St. on the NW corner.
Not only was there a live action TV sitcom spinoff of this; there was a cartoon spinoff called "The Oddball Couple" with a cat and a dog playing Felix and Oscar; called "Fleabag" and "Spiffy": in this version.
The pilot episode of the 2015 TV reboot of The Odd Couple, starring Matthew Perry (Oscar) and Thomas Lennon (Felix), used a good chunk of this movie as its plot; Oscar offered to let Felix move in, cleanliness of apartment is like night and day, chases Oscar's friends out with his cooking and other things, Oscar setting up a double date with two sisters in the same building, Felix commandeering the date and Oscar making the girls mad at him, etc.