A graduate from Hyundai High School in Seoul, in 2018 he signed his first professional contract with K League 1 team Ulsan Hyundai. He was originally set to reinforce the youth squad of the "Horangi" in the national reserve league: however, on March 1, 2018, Ulsan's manager Kim Do-hoon caught everyone by surprise by giving Oh his senior debut at only 19 years old, as he chose him for the starting XI against Jeonbuk Hyundai Motors in the first game of the season. He played for 56 minutes before being replaced by Júnior Negrão, and his side eventually suffered a 2-0 away loss.