O'Hara is one of the cats in the group of kittypets that have been harassing ShadowClan. Bramblestar leads a patrol to take on the cats, since he can't let his sister's Clan suffer, as well as protecting their own territory.
They appear behind the group after they encounter Webster, a friend of Victor's. O'Hara waves his tail at Bramblestar, purring that it was nice to meet him, before all four of the kittypets launch at Bramblestar and Jessy. O'Hara knocks Bramblestar down as he crashes into the leader, and Bramblestar rakes his claws down the silver kittypet's side, and O'Hara backs off.
He trips over Lionblaze before crouching on top of the tom and rakes his claws across Lionblaze's shoulders, until Cinderheart drags the kittypet off her mate and chases him off.