In 1996, he won both a Gold Glove Award and the MVP of the KBO with a 1.88 earned run average, 18 wins, 24 saves and 183 strikeouts in 139 innings pitched.

The Orix BlueWave merged with the Osaka Kintetsu Buffaloes in 2004, and he took the opportunity to announce his decision to play in the major leagues.

This came right when Fox TV analyst Tim McCarver commented on how Koo's plate appearance was "the biggest give-up at-bat".

On a 1-and-1 count, Koo crushed a deep line drive right over Bernie Williams's head one hop off the wall in right-center field.

Upon retrieving the thrown ball from first base, Posada applied the tag onto Koo's left shoulder as he slid head first towards home plate on a close play.

Home plate umpire Chuck Meriwether called Koo safe on the play, extending the Mets' lead to 3-0.

In a press conference on August 22, 2010, Koo revealed he was to play for the Sydney Blue Sox in the Australian Baseball League.

With his team up 1–0 against the Canberra Cavalry, he recorded the final out by striking out Nick Kimpton to earn the first save in the Australian Baseball League.

However, in the 2013–14 season, Williams moved to the Adelaide Bite and Koo was once again named Sydney's closer, posting a stellar 2.08 earned run average with 11 saves.

[6] Koo was selected for the Australian national baseball team in two spring training exhibition matches against the Los Angeles Dodgers and the Arizona Diamondbacks.

[7] After managing Geelong-Korea for two seasons, Koo came out of retirement, pitching an inning, at 48 years old — making him the oldest player to have played in the Australian Baseball League.