Oku turned professional after graduating from Kobe Koryo Gakuen High School in 1994. In 280 J. League matches, representing Jubilo Iwata, Yokohama F. Marinos and Yokohama FC, he scored 62 times. He played for Jubilo until 2001, helping the club win two league titles and then played five seasons with Yokohama F Marinos, where he won two more titles. He finished his career in 2007 after playing for one season with Yokohama.
Oku earned his first national cap in 1998 and appeared in 26 matches through 2004 for the senior national side, scoring on two occasions.
Oku married actressHinako Saeki in 2002. In 2013, Oku was arrested by Kanagawa Prefecture police after he threatened to kill Hinako on multiple occasions. Saeki eventually filed for divorce later that year. But prosecutors decided not to indict Oku.
On the morning of 17 October 2014, Oku was killed following a car accident on Miyako Island. He was driving on a prefectural road at about 4:25 a.m. when his car veered into the opposite lane and struck a telephone pole. According to police, he was transported to a hospital, where he was soon pronounced dead from a broken pelvis and other injuries.There were no other passengers in the car and following his death, police began investigating the cause of the accident.