His first Star Trek appearance was in 1989 and his last was in 2003. He played Gowron for nine years. When he first auditioned for the role, he attempted to portray him as cold and calculating, but feeling this was not as successful, at his next audition, he flared his eyes aggressively (which Gowron has now come to be known for) and successfully obtained the role.

In addition, O'Reilly appeared as Kavok, the antagonist of the Star Trek The Next Generation - A Klingon Challenge interactive board game, and portrayed Gowron's ancestor Aakar in Star Trek Online's Star Trek Discovery-based storylines beginning with the Season 20 update (Aakar's character model was also updated to resemble Gowron).