Following his time with the Athletics, O'Neill played in 16 games with the minor league Harrisburg Senators and also played semi-pro basketball and football. After the outbreak of World War II, O'Neill enlisted in the United States Marine Corps in 1942, and rose to the rank of first lieutenant with the Weapons Company, 25th Marine Regiment, 4th Marine Division. In January 1944, he took part in the amphibious assault on Kwajalein. On June 16, 1944, the second day of the Battle of Saipan, he was wounded in the shoulder by shrapnel, then treated for weeks back in the US. He returned to active duty in July, in time to participate in the Battle of Tinian. He was killed by a sniper at Iwo Jima on March 6, 1945. Among his surviving family was his young wife, Ethel McKay O'Neill.