Basically a line-drive hitter and a good fielding replacement, O'Brien entered the majors in 1943 with the Pittsburgh Pirates, playing for them three years before joining the Boston Red Sox (1949–1950) and Washington Senators (1950). His most productive season came in his rookie year, when he posted career-highs in batting average (.310), runs (35), extrabases (21), RBI (26) and games played (89).
