O'Neill was an outlaw gorilla from an Undefined Multiverse. Kidnapped from the jungle as a child, O'Neill was brought to the United States, where he was adopted by prospector Bart Masters.
Masters was a loving and caring master, who trained O'Neill to survive in the Old West. Unfortunately, this was to be fortunate, as Masters was murdered by Tutt Strawhan, leader of the Strawhan Gang, in order to learn what Masters knew about a "great mother-load." O'Neill swore revenge on his master's killers, setting out armed and dangerous to wipe out the Strawhan Gang.
Armed with only a full bandolier and two six-shooters, He tracked down and killed the members one by one. Sadly, the Six-Gun Gorilla died while killing Tutt Strawhan.
His story was repeated for quite some time.