 While attending Cheongwon High School in Seoul, Oh was considered one of the top lefty high school pitching prospects.In , 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.Upon graduation from Cheongwon High School in Seoul, Oh was drafted by the Hyundai Unicorns in the 2nd round (12th overall) of the KBO Draft.