O'Neill is a veteran of the Edinburgh Fringe Festivals, having performed stand-up or theatre gigs there for over twenty years. He was nominated for the 1994 Perrier Award with his show "It's a Bit Like This", and won a Fringe First in 1999 with Sean Hughes for the theatre show "Dehydrated and Travellin' Light". Theatre sets have included 12 Angry Men, One Flew Over The Cuckoo's Nest and The Odd Couple. Stand-up has included "Off My Face" and "It Was Henry Fonda's Fault".