John O. Barnum (October 6, 1911 – October 30, 1996) was an American professional golfer who played on the PGA Tour in the 1950s and 1960s. Barnum is notable for two significant milestones in golf. He was the second golfer (after Jim Barnes), and one of only seven, ever to win a PGA Tour event after turning 50, and the only player to win his first PGA Tour event past the age of 50. His best finish in a major was T-16 at the 1958 PGA Championship.