Oku was arrested by police on June 6 on suspicion of calling his wife and telling her, "I'm going to come and kill you."
The Yokohama District Public Prosecutors Office did not say on Thursday why it has decided not to indict Oku, Kyodo news agency reported.
The 37-year-old Oku was capped 26 times for Japan and scored twice. He played 280 games in the J-League, mostly with Yokohama F Marinos and Jubilo Iwata.
After retiring in 2008, Oku served as a director of development with Yokohama but left the club in January due to poor health.
Oku's wife, actress Hinako Saeki, said she will drop her complaint and accept an out-of-court settlement. The couple, married in 2002, is negotiating a divorce.