He has writing credits on some of Cannibal Corpse's more complicated songs, including "Hatchet to the Head," "Systematic Elimination," "Frantic Disembowelment" and "Blunt Force Castration." He is featured on the albums Gallery of Suicide, Bloodthirst, Gore Obsessed, Worm Infested, The Wretched Spawn, Kill, and Evisceration Plague.
Pat O'Brien joined Cannibal Corpse in 1997 after recording In Memory and The Politics of Ecstasy with Nevermore in 1995 - 1996, replacing Rob Barrett (2) on guitars. He also played in Cincinnati based death/thrash metal band Ceremony (6) for a while in the 80's as well as David T. Chastain, Prizoner, and Monstrosity. In 2011, Pat played some live dates with Slayer, filling in for an ailing Jeff Hanneman.