In April 1995, Oku was selected teh Japan U-20 national team for 1995 World Youth Championship. At this competition, he played all 4 matches and scored a goal against Brazil in Quarterfinals. After 1998 World Cup, in October 1998, Oku was selected the Japan national team by new manager Philippe Troussier. On October 28, he debuted for Japan against Egypt. He played at 1999 Copa América and 2000 Asian Cup. At 2000 Asian Cup, he played 4 matches and Japan won the champions. However he was not selected Japan for 2002 World Cup. In 2003, he was selected Japan for the first time in 2 years by manager Zico. He also played at 2003 Confederations Cup. He played 26 games and scored 2 goals for Japan until 2004.