O'Malley helped lead Southridge High School to the 2003-2004 Washington State 4A Baseball Title. O'Malley received all-state honors in football and baseball. He committed to play baseball at Washington State University.
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.[2]
Seattle Mariners
On January 22, 2015, O'Malley was signed by the Seattle Mariners to a minor league contract with an invite to Major League spring training. 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.[3]
In his first game with the Mariners, he went 3 for 4 with three singles, two RBI, and a stolen base in an 8-3 win over the Houston Astros. O'Malley finished the 2015 season batting .262 with Seattle.
Despite a very successful Spring Training, batting .471 (16-for-34) in 21 games, O'Malley was optioned to Triple-A Tacoma to begin the 2016 season. O'Malley was recalled by the Mariners on May 15, 2016, after batting .317 in 25 games with the Tacoma Rainiers.[4]
On June 2, 2016, O'Malley played a vital role in the Mariners biggest comeback in club history. He drove in the game winning run with an RBI single.[5] He missed the entire 2017 season due to injury. He elected free agency following the season on November 6, 2017.[6]
Colorado Rockies
On December 16, 2017, O'Malley signed a minor league contract with the Colorado Rockies.[7] In 55 games for the Triple–A Albuquerque Isotopes, he batted .279/.321/.367 with one home run and 16 RBI. O'Malley elected free agency following the season on November 2.[8]
Kansas City T-Bones
On February 27, 2019, O'Malley signed with the Kansas City T-Bones of the independent American Association. He retired from professional baseball following the season. In 83 games he hit .307/.378/.429 with 5 home runs, 40 RBIs and 10 stolen bases.
Coaching career
O'Malley was announced as hitting coach for the Modesto Nuts, the High–A affiliate of the Seattle Mariners, for the 2020 season.[9] He was promoted to be the hitting coach of the Double-A affiliate Arkansas Travelers for 2023.
On January 24, 2024, O'Malley was announced as the hitting coach of the Triple–A Tacoma Rainiers.[10]