In 2003, he was selected for the South Korea national junior baseball team and competed in the 5th Asian Junior Baseball Championship held in Bangkok, Thailand, where South Korea won its second Asian Championship, beating Chinese Taipei in the final game.
After the season, he joined the Korea Armed Forces Athletic Corps Baseball Team to serve two years of mandatory military service.
In 2009, Oh returned to the pro league after finishing the military duty but had a mediocre comeback season, posting a 1-2 record and 5 holds with a 4.08 ERA as a relief pitcher.
In 2010, Oh played a vital role in the Heroes' bullpen, appearing in a team-leading 69 games (4th in the league).
In 2011, Oh had the best season as a setup man, racking up a career-high 20 holds which ranked him third behind Jung Woo-Ram's 25 and Jong Hyun-Wook's 24.