Nagayasu was the son of a sarugaku player for the Takeda clan. He became a minor administrator for the Takeda and was later adopted by Okubo Tadachika (from whom he adopted his surname) and became the commissioner of mines for Tokugawa Ieyasu after 1590. In this role he proved most useful to Ieyasu, though he was suspected of fraudulent activities. Nonetheless, he was given a 30,000-koku fief at Hachijo (Musashi province) and in 1606 was made daikan of Izu, handling tax collection and finances in general for that province. Such was his importance; he was nicknamed Tenka no Sôdaikan, or 'Great Administrator of the Realm'. He became involved in a bitter feud with Honda Masazumi that worsened the fortunes of the Okubo in general. After his death in April 1613, Nagayasu's illegal activities came to light and his family was harshly punished.
- Initial text from Samurai-Archives.com FWSeal & CEWest, 2005