The Yankees traded O'Brien to the Arizona Diamondbacks for Martín Prado on July 31, 2014. The Diamondbacks assigned him to the Mobile BayBears of the Class AA Southern League. After the 2014 season, the Diamondbacks assigned O'Brien to the Arizona Fall League. The Diamondbacks assigned O'Brien to the Reno Aces of the Class AAA Pacific Coast League (PCL) in 2015, and converted him into an outfielder. On July 14, 2015, O'Brien participated in the Triple A Homerun Derby. He finished in second place with a total of 20 home runs.
