For TRUE Odd Couple fans ONLY: HOW DID OSCAR AND FELIX MEET?

since you didn't post this in the movie forum I assume you're asking about the Klugman/Randall show.
The short answer is-it's never made clear; as the show gives 3 or 4 different ways

That was why you heard the phrase "childhood friend" in the opening narration. When it was discovered in a flashback episode that Felix and Oscar met on jury duty, the word childhood was deleted.