Sydney Lloyd O'Brien (born February 18, 1944) is a former infielder in Major League Baseball. He played more games at third base than any other position, but also played a significant number of games at shortstop, second base, and first base. He batted and threw right-handed.
O'Brien was signed by the Boston Red Sox as an amateur free agent in 1964, but was drafted out of the minors by the Kansas City Athletics in November 1964, only to be traded back to Boston two years later. He made his major league debut for Boston in 1969, pinch hitting for Fred Wenz in the bottom of the eighth inning in a game against the Baltimore Orioles. He finished the season with five triples, ranked ninth in the American League.
After the 1969 season, the Red Sox traded O'Brien to the Chicago White Sox. He played in 121 games for the White Sox in 1970, the most he would play in a single season, with career highs in batting average, hits, runs batted in, runs scored, walks, and steals, while also pacing the poor-fielding club with a .948 fielding percentage. After the season, the White Sox sent O'Brien to the California Angels, where he would play a season and a half primarily as a shortstop. In the middle of the 1972 season, he was traded to the Milwaukee Brewers, where he finished his career.