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.

Some weeks before, on August 2, during an away match against Gangwon, a collision between him and Jung Ji-yong during a header attempt left the latter with a concussion, causing him a loss of consciousness for some minutes: Oh, who got booked following the impact, was visibly shocked.

Even though bad luck kneecapped his season, Oh still managed to register 13 appearances (all as a starter) and four goals, contributing to the Sangmu's highest finish ever in the top-tier league, with the club obtaining a final fourth place despite being destined to automatic relegation since the start of 2020, due to their planned re-location.

Right after he completed his military service and came back to Ulsan Hyundai, manager Hong Myung-bo included him in the list of players that would travel to the centralized venue in Bangkok and take part in the group stage matches of the AFC Champions League throughout June and July 2021.

Being one of the youngest members of the squad, he was used a substitute, but still managed to leave his mark in the competition: coming in for Lee Seung-woo in the injury time of the group stage match against Guinea, he scored the winner just one minute since he had got on the pitch, as he sent the ball in the top corner of the net after being assisted by You Ju-an.

In 2018, he took part both in the Toulon Tournament, where the South Korean selection was eliminated in the group stage,[13] and the AFC Under-19 Championship, where his side reached the final before losing 1–2 to eventual winners Saudi Arabia.

[14] In 2019, he was selected by head coach Chung Jung-yong to take part in the FIFA U-20 World Cup in Poland, being the only player in South Korea's 21-men list who was serving his military duty.

Except for the debut in the group stage against Portugal, Oh started every single match of his side, scoring two consecutive headers, against Argentina and Japan (in the round of 16),[2] and one penalty in South Korea's successful shoot-out against Senegal in the quarter-finals.

Used frequently during the tournament, he scored a brace in South Korea's 2–1 win against Uzbekistan in the last game of the group stage,[21] thus getting involved in the Taeguk Warriors' first victory of the competition, as they beat Saudi Arabia after the extra-time in the final and qualified for the Olympics.