William John O'Neill (January 22, 1880 – July 20, 1920) was an outfielder in Major League Baseball who played for the Boston Americans (1904), Washington Senators (1904) and Chicago White Sox (1906). O'Neill was a switch-hitter and threw right-handed. He was born in Saint John, New Brunswick, Canada. In a two-season career, O'Neill was a .243 hitter with two home runs and 42 RBI in 206 games played. O'Neill died in Woodhaven, New York, at the age of 40.