Oku was born in Amagasaki on February 7, 1976. After graduating from Kobe Koryo Gakuen High School, he joined J1 League club Júbilo Iwata in 1994. He debuted in 1996. He became a regular player from 1997. The club won the champions at 1997, 1999 J1 League and 1998 J.League Cup. In Asia, the club won the champions 1998–99 Asian Club Championship and the 2nd place 1999–00 and 2000–01 Asian Club Championship. He moved to Yokohama F. Marinos in 2002. The club won the champions in 2003 and 2004 J1 League. He also played as captain and was selected Best Eleven in both seasons. He moved to Yokohama FC in 2007. He retired end of 2007 season.