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).

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.

[1] In the midseason he was traded to Washington in the same transaction that brought Kip Selbach to Boston.

In a two-season career, O'Neill was a .243 hitter with two home runs and 42 RBI in 206 games played.