Sydney Lloyd O'Brien (born February 18, 1944) is an American former professional baseball player.

He batted and threw right-handed and was listed as 6 feet 1 inch (1.85 m) tall and 185 pounds (84 kg).

He then attended Long Beach City College and 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.

At the age of 25, O'Brien 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 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 (RBI), runs scored, walks, and steals, while also pacing the poor-fielding club with a .948 fielding percentage.