Ernest Borgnine and Tim Conway, who starred in McHale’s Navy, have recurring roles as Mermaid Man and Barnacle Boy on SpongeBob Squarepants.
I believe Joanna Lumley make a few appearances with Jennifer Saunders and Dawn French on French and Saunders, and then starred in Absolutely Fabulous with Jennifer Saunders. Julia Sawalha also appeared on French and Saunders as well as starring in AbFab. And all three of them appeared in Mirrorball.
Harold Perrineau and Adewale Akinnuoye-Agbaje (sheesh, what a name) starred together on “Oz” as Augustus and Adebisi. They later both starred on “Lost” as Michael “WAAAALT” Dawson and Mistereko.
Harol Perrineau and Terry Kinney starred together in “Oz” and on the recent “The Unusuals.”
Another “Oz” one: Lauren Velez, David Zayas, and Erik King all played prominent characters on both “Oz” and “Dexter.”
Hey, I’m good at this game!
Ooh, thought of another “Oz” one: J.D. Williams (Kenny) and Seth Gilliam (Clayton Hughes) first starred together on “Oz” and then went on to bigger roles as Body and Carver on “The Wire.”
So basically, everybody has starred on “Oz” at some point or another.