I just saw the Jury Duty episode, how many others were there?

In a flashback episode, Felix and Oscar's Fathers (played by Randall and Klugman) had an adventure together, and in the final scene young Felix and young Oscar (played by child actors) meet.

I believe there was another flashback episode where they met while they were in the Army.

"This is the Army, Mrs. Madison" was the episode that showed Felix and Oscar meeting and Oscar proposing to Blanche.