O'Hare died Friday, five days after his heart attack, The Hollywood Reporter reported.
"This is a terrible loss for all B5 fans, and everyone involved with the show wishes to convey their condolences to the O'Hare family," series creator J. Michael Straczynski wrote on his Facebook page. "He was an amazing man."
The Chicago native starred as Cmdr. Jeffrey Sinclair on the first season of the sci-fi series "Babylon 5," and returned for three episodes in later seasons to wrap up his character's storyline.
O'Hare also acted in "The Cosby Mysteries," "Kate & Allie," "Trapper John, M.D.," "The Equalizer," "Law & Order, L.A." "Law and Tales From the Darkside" and the daytime soap "One Life to Live."