O'Malley helped lead Southridge High School to the 2003-2004 Washington State 4A Baseball Title. O'Malley was drafted by the Tampa Bay Devil Rays in the fifth round of the 2006 Major League Baseball Draft (139th overall) out of Southridge High School in Kennewick, Washington. He played in the Devil Rays/Rays organization until 2013 without reaching the majors.
Before the 2014 season he signed a minor league deal with the Los Angeles Angels of Anaheim. O'Malley was called up to the majors for the first time on September 1, 2014. He made his major league debut on September 7, 2014, when he pinch-hit for Albert Pujols at Minnesota against Twins pitcher Ryan Pressly, reaching on an infield hit to shortstop which drove in a run.
O'Malley was released by the Angels on December 19, 2014. O'Malley was signed by the Seattle Mariners to a minor league contract with an invite to Major League spring training on January 22, 2015. While batting .345 at triple-A with the Tacoma Rainiers, he was hindered from getting promoted due to a wrist injury, until rosters expanded.
The Mariners promoted him to the major leagues on September 1, 2015. O'Malley finished the 2015 season batting .262 with Seattle.