In the set-up role, Ko drew the attention of the league when he posted a 1.95 ERA and earned 16 holds in 2009. He enjoyed another great season as a setup man in 2010, when he earned 22 holds which was runner-up in the KBO league.  After the 2010 season, Ko was selected for the South Korean national baseball team to compete in the 2010 Asian Games where he won the gold medal. 
In 2011, Ko earned 14 holds but his ERA rose to 4.44. In 2012, he finished the season with a 8.62 ERA losing all ability to control his pitches. Ko was sent down to the second-tier team of the Bears in the middle of the 2012 season and traded to the NC Dinos after the season.