Kim Sung-oh made his acting debut in the play First Love in 2000. Two years later, he landed a minor role in the satirical film Emergency Act 19 and has since appeared in numerous TV series and movies. His career breakthrough came in 2010 when he became part of the blockbuster action-thriller film The Man From Nowhere. That same year, he joined the hit SBS dramas Giant and Secret Garden. His portrayal of Hyun Bin’s bumbling secretary in the latter earned him a Best New Actor nomination at the 47th Baeksang Arts Awards. In 2016, he starred in the four-episode KBS2 drama Baek-hee Has Returned, which won him an Excellence Award at the KBS Drama Awards. In 2017, he played Park Seo-joon’s coach and friend in KBS2’s Fight My Way, which gave him the Best Supporting Actor trophy at the KBS Drama Awards. In 2020 and 2021, he was cast as one of the main characters in JTBC’s Graceful Friends and tvN’s L.U.C.A.: The Beginning.

Kim Sung-oh attended Kyungbok University. He married his non-celebrity girlfriend in December 2014. They have a son together.