Born in Seoul, South Korea, Ou began his football career at Yonggang Middle School and entered the Korea Football Association Outstanding Players Study Abroad Project, resulting in him moving to France by joining Ligue 1 side FC Metz, along with Kang Jin-wook as part of the club's partnership with the project.
[6] He made his FC Metz debut, coming on as a late substitute, in a 3–1 loss against OGC Nice on 20 October 2007, which turns out to be his only appearance for the club.
[7] During his time there, Ou began learning French but struggled despite spending five years in the country and later his mother and younger brother joined him in France.
[11] Upon joining the club, Lee Young-jin said of the move: "As a player who has been watching Seoul with interest early, he has steadily checked his condition even after entering France.
[22] After his loan spell at Daejeon Citizen came to an end, Ou returned to FC Seoul and soon received more playing time at the start of the 2011 season.
[24] Ten days later on 15 March 2011, Ou scored his first FC Seoul goal in the AFC Champions League match against Zhejiang Greentown, which resulted in the club winning 3–0.
Ou was involved in a related 2011 match fixing scandal and was officially announced in August 2011 that he would not be able to play in the all league systems in South Korea permanently.