Handsome American leading man of classic silent films who became a different kind of star in B-Western talkies. The son of a policeman who later became police chief of San Francisco and then California Director of Penology, O'Brien was raised around police stables and quickly became adept at horsemanship.
He was a star athlete in school and intended to study medicine, but with the advent of World War I, he joined the Navy and volunteered to serve as a stretcher-bearer with the Marines. Following his discharge, a chance encounter with Tom Mix led to a job as camera assistant with Mix's production company. This in turn led to small jobs as a prop man, extra, stuntman, and finally bit player.
John Ford spotted the husky young man and cast him in the lead role of his early Western The Iron Horse (1924). He continued to work for Ford and became a popular leading man for a number of top-flight directors. With the coming of sound, he moved almost exclusively into Westerns and became a popular star of low-budget oaters.
At the outbreak of World War II, O'Brien reenlisted in the Navy, served for a time as a recruit trainer, then participated in numerous island invasions in the Pacific Theater and was highly decorated. He played a few roles, particularly for Ford, after the war, but returned to naval duty in the Korean conflict and again during the Vietnam war.
He left service with the rank of captain, having four times been recommended for the rank of admiral. He spent his later years ranching, but following a heart attack, was confined to bed for the last few years of his life. He died in 1985.