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.

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.

O'Brien died later that year of typhoid fever at the age of 29.

This biographical article relating to an American baseball outfielder born in the 1860s is a stub.