John Thomas O'Brien (born December 11, 1930 in South Amboy, New Jersey) is a former backup second baseman and pitcher in Major League Baseball who played for the Pittsburgh Pirates (1953, 1955–58), St. Louis Cardinals (1958) and Milwaukee Braves (1959). O'Brien batted and threw right-handed. His twin brother, Eddie, is a former major league shortstop.

O'Brien attended Saint Mary's High School in South Amboy, now Cardinal McCarrick High School, where he has been inducted into the school's sports hall of fame.

O'Brien attended Seattle University, where he played on the basketball team for the Chieftains (along with his brother Eddie) and scored 43 points in a stunning 84–81 upset over the Harlem Globetrotters on January 21, 1952. O'Brien would be the shortest NCAA All-American player to be named until 2016 when the similarly 5 ft 9 in (1.75 m) Tyler Ulis would be named a member of their first team. Later he and Eddie were drafted by the NBA's Milwaukee Hawks in 1953, but the twins never played in the NBA.

In a six-season career, O'Brien was a .250 hitter (204-for-815) with four home runs and 59 RBI in 339 games played. From 1956–58, he also doubled as a pitcher, appearing in 25 games (all but one in relief) and 61 innings, surrendering 61 hits, walking 30 and striking out 35. He lost three of four decisions (.250) with an earned run average of 5.61.

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