William O'Brien enlisted in Ballarat, Victoria at the age of 24 on the 28th of April 1915. He had been born in Smythesdale, Victoria was single, and was employed as a Wringer at the Woollen Mills. He volunteered and was stationed with the 58th Battalion, 9th Reinforcement later transferring to the 6th Battalion with the Regimental number 7549. He was discharged and Returned to Australia on the 13th of July 1919. Tree number 1117.