John Thomas O'Brien (born December 11, 1930) 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).

[1] 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.

From 1956 to 1958, 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.

While in Pittsburgh, Johnny and Eddie O'Brien became the first twins in major league history to play for the same team in the same game.

The others are Garvin and Granny Hamner, for the Philadelphia Phillies in 1945; Frank and Milt Bolling, with the Detroit Tigers in 1958, and Billy and Cal Ripken, for the Baltimore Orioles during the 1980s.