William Smith O'Brien (March 14, 1860 – May 26, 1911) was a Major League Baseball first baseman. He was a native of Albany, New York.
O'Brien played for the St. Paul Saints and Kansas City Cowboys, both of the Union Association, in 1884. He also played for the National League Washington Senators (1887–1889) and the Brooklyn Gladiators (American Association) (1890). His best season was 1887 when he led the National League in home runs with 19. (Hall of Fame first baseman Roger Connor finished second with 17.) He also led his own team that year in runs batted in with 73.
O'Brien's career totals include 356 games played, 364 hits, 32 home runs, 206 RBI, and a lifetime batting average of .256. He even had a 1–0 record as a pitcher, appearing twice in relief for the Saints during his rookie season.
He died in 1911 in Kansas City, Missouri.