William O'Brien (1850 – unknown), better known as Billy Porter but also known by the alias William or Billy Morton, was an American burglar and underworld figure in New York City during the mid-to late 19th century. He and partner were longtime members of the Dutch Mob along with Little Freddie and Michael "Sheeny Mike" Kurtz. He was present during the 1883 gunfight at Shang Draper's saloon in which Irving was shot and killed by rival John "Johnny the Mick" Walsh. O'Brien then killed Walsh and was himself gunned down by Shang Draper. Although surviving his wounds, he was tried for, and acquitted of, Walsh's death.