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 his victory at the Cajun Classic Open Invitational. He was the second golfer, after Jim Barnes, 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.
Barnum won five Michigan PGA Senior titles and was runner-up three times in the PGA Seniors' Championship (1963, 1964, 1966).