His prowess as a relief pitcher also earned him the nickname "Kkeut-pan Wang" (Final Boss), in South Korea and among Cardinals fans.

He joined the 2001 KBO draft at the end of his last high school season, but, despite intriguing some MLB scouts, went undrafted, and Oh elected to attend college instead.

Upon graduation from high school, Oh started his collegiate career at Dankook University, but missed the entire 2001 and 2002 seasons after undergoing Tommy John surgery in 2001.

[citation needed] The Samsung Lions selected Oh in the second round (5th pick, 12th overall) of the 2005 KBO First Year Players draft.

In the 2005 Korean Series, he pitched seven scoreless innings with 11 strikeouts, appearing in three games as a closer to lead his team to the championship.

The Lions defeated the Fukuoka SoftBank Hawks, the Nippon Professional Baseball champions, to earn the Asia Series championship in 2011, and become the first non-Japanese team to win.

[4] Tigers' manager Wada Yutaka suggested offering some Korean-language classes for his players to better communicate with their new teammate, especially the catchers and pitching coaches.

His fast and heavy balls, together with his calm and unwavering expression even in crisis situations, earned him a new nickname "Stone Guardian" from Japanese baseball fans.

Through eleven total professional seasons in South Korea and Japan, he recorded 357 saves, a 1.81 ERA, and 10.7 strikeouts per nine innings in 646+1⁄3 IP.

[13] On January 11, 2016, Oh signed a one-year contract with the St. Louis Cardinals of Major League Baseball (MLB),[14] with a club option for a second season.

[16] In May, Oh began to receive attention as a top candidate for the National League (NL) Rookie of the Year Award.

[22] On February 26, 2018, Oh signed a one-year, $2 million contract with the Toronto Blue Jays that included a vesting option for the 2019 season.

[23] On July 26, 2018, the Blue Jays traded Oh to the Colorado Rockies for Forrest Wall, Chad Spanberger, and a player to be named later or cash considerations.

[25] On August 5, 2019, Oh returned to Korea and signed with the Samsung Lions of the KBO League, the team he began his professional career with.

[32] The Seoul Central District Prosecutors Office fined Oh and Lim in January 2016,[33] and the KBO League suspended Oh for six months.

[31] With Japanese law toward gambling among athletes similarly restrictive, the Hanshin Tigers terminated his contract amid reports he was linked to a Korean crime organization.