Tracy enlisted in the United States Navy when he turned 18. He was sent to the Naval Training Station in North Chicago, where he was still a student when World War I came to an end. He achieved the rank of seaman second class, but never went to sea, and was discharged in February 1919.
O'Brien attended Marquette Academy with fellow actor Spencer Tracy, who became a lifelong friend. During World War I, O'Brien and Tracy joined the United States Navy. They both attended boot camp at the Great Lakes Naval Training Center, but the war ended before their training had finished.
Jack Benny was also at the Great Lakes Naval Training Center at the same time as O'Brien and Tracy. According to his autobiography, Benny performed a number on the violin at a show one evening, when the sailors started booing and heckling him. O'Brien walked on stage and whispered in his ear, "For heaven's sake, Ben, put down the damn fiddle and talk to 'em." Benny stopped playing his violin and made a series of comments that got laughs from the audience. In this way, O'Brien indirectly helped to start Benny's career in comedy
Gentlemen: we salute you and thank you for your willingness to serve your country. Rest in peace.