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.
In 1911, he had a record of 26–7 while playing for the Denver Grizzlies in the Western League, leading the league in winning percentage and strikeouts. The Grizzlies won 111 games en route to their championship. In September of that year, O'Brien made his debut in Major League Baseball with the Red Sox, going 5–1 with a 0.38 earned run average.