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, O'Brien was a standout pitcher for the Denver Grizzlies, leading the league in wins and strikeouts while helping them win their championship. He then made his Major League debut with the Red Sox that same year, achieving a strong record of 5–1 with a low ERA.