 A valuable utility man, O'Brien was able to play all positions except pitcher and catcher, although he played mostly in the outfield or at first base.He reached the majors in 1897 with the Baltimore Orioles, spending one and a half years with them before moving to the Pittsburgh Pirates (1898), New York Giants (1899), and again with Pittsburgh (1900).His most productive season came in 1899 with the Giants, when he posted career-highs in batting average (.297), home runs (6), RBI (77), runs (100), and games played (150).