William Smith O'Brien (March 14, 1860 – May 26, 1911) was an American Major League Baseball player who played first base.

He started his professional baseball career in 1884 and spent most of the season with the St. Paul Apostles of the Northwestern League.

[1] In September, O'Brien made his major league debut in the Union Association, playing a total of 12 games for the St. Paul Saints and Kansas City Cowboys.

Just like in 1884, the addition of a third major league allowed him to receive playing time in the worst of the three circuits.

[1] In 356 career MLB games, O'Brien had a .256 batting average, 32 home runs, 206 RBI, and a 107 OPS+.