O-Four, designated by the numerical identifier RC-1304, was a male Human clone derived from the genetic material of the Mandalorian bounty hunter, Jango Fett. He was bio-engineered by the Kaminoan cloners situated on the planet Kamino, with the intention of integrating him into the Grand Army of the Galactic Republic. Trained as a clone commando, O-Four became part of Niner's squad, fighting alongside commandos DD and Sev under the leadership of sergeant Niner. The unit was dispatched to the planet Geonosis during the first battle of the Clone Wars against the Confederacy of Independent Systems. During this engagement, O-Four, DD, and Sev all perished, leaving Niner as the sole survivor of Lambda. The sergeant later reflected that deploying O-Four westward with an E-Web heavy repeating blaster during the conflict would have been a better decision. Like all clone troopers, O-Four's height was 1.83 meters.
The character O-Four was initially referenced within Republic Commando: Hard Contact, a novel from 2004 authored by Karen Traviss. Subsequent mentions of him appeared in Republic Commando: Triple Zero, another installment within the same series.