William D. "Darby" O'Brien (September 1, 1863 – June 15, 1893) was a Major League Baseball player in the late 19th century. He played outfield for the New York Metropolitans in 1887 and the Brooklyn Bridegrooms/Grooms from 1888–1892. O'Brien developed lung problems during his playing career and continued to play, despite his ill health. When he reported to spring training for the 1893 season, the team found that he was too ill to play and sent him to Colorado to try to recover. They played a benefit game to raise money for him.