He was in the David Letterman show almost since the beginning. He invented the Late Show Top Ten List while he was head writer of the show, and he wrote the lists is collaboration with the other writers of the show. He later co-authored several of the Top-ten books based in the show.Sources for top-ten list:
He has been the head writer of Jimmy Kimmel Live since the show's debut in January, 2003. He occasionally appears acting in comedy bits for the show in front of camera as well.
He is the identical twin of Mark O'Donnell, the Tony-award winning co-writer of the Broadway musical, Hairspray.
He won Primetime Emmy Awards in the "Outstanding Writing in a Variety or Music Program" category on four occasions: for Late Night with David Letterman in 1984, 1986 and 1987 and for The Chris Rock Show in 1998.