Roark's Onix was Roark's second most used Pokemon.
Onix was first used to battle Paul's Elekid and Chimchar in Shapes of Things to Come. Onix had the advantage due to his large size and immunity to Elekid's electric attacks. Elekid managed to paralyze Onix with Static but Onix used Stealth Rock, Paul recalled Elekid and replaced it with Chimchar. Chimchar avoided Onix's attacks by using Dig and defeated Onix.
Onix appeared again when Roark battled Ash, easily defeating Turtwig with a single Double-Edge. Onix then battled Pikachu and won by unleashing a Screech, blowing Pikachu into a rock, defeating him and winning the battle.
Onix battled Pikachu a second time during Ash and Roark's rematch. During the battle Pikachu used a Thunderbolt that was powerful enough to actually hurt Onix. Stunned by Pikachu's incredible electrical power, Roark's reacted too slowly to form an effective counterstrike and Pikachu lunged forwards and smashed Onix with Iron Tail, knocking him out.