Thomas Joseph "Buck" O'Brien (May 9, 1882 – July 25, 1959) was a starting pitcher in Major League Baseball who played for the Boston Red Sox and Chicago White Sox. O'Brien got a late start in professional baseball. However, as a spitballer, he did have tremendous success for a few years. In 1910, when he was 28 years old, he went 20–10 for Hartford of the Connecticut State League. In 1911, he was 26–7 for the Western League's Denver Grizzlies, leading the league in winning percentage and strikeouts. The Grizzlies won 111 games en route to the league championship. In September of that year, O'Brien made his major league debut with the Red Sox and went 5–1 with a 0.38 earned run average.