He played for the San Francisco Giants, Chicago White Sox, Baltimore Orioles, Texas Rangers, Montreal Expos, and New York Mets.

He was quickly picked up by the Yakult Swallows in 1995, and played his best season that year, hitting 31 home runs with 87 RBIs and a .307 batting average.

He hit .300 for the sixth season in a row in 1996, (a record among non-Japanese players in the Japanese professional leagues) but split with the Swallows over contract obligations.

He was also a mentor for non-Japanese players, including George Arias, Jeff Williams, Trey Moore, and Jerrod Riggan, who contributed to the Tigers' championship in 2003.

He has appeared in various television commercials in the local Kansai region (Hanshin Electric Railway Co., Ltd.), and posters featuring O'Malley could be found in Koshien Stadium.