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.
Playing at shortstop in his 1904 rookie season for the Red Sox, O'Neill committed six errors during a 13-inning 5–3 loss to the St Louis Browns on May 21 to become the only 20th-century Major League player to record six errors in a game. In the midseason he was traded to Washington in the same transaction that brought Kip Selbach to Boston. In 1906 O'Neill was a member of the Chicago White Sox team that won the World Championship over the Chicago Cubs in six games.
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.