小田原藩
Ōkubo was born to a "samurai" family; his father was descended from the Ōkubo clan, former "daimyō" of Odawara Domain, who served as hereditary Shinto priests at a shrine in Tōtōmi Province (in what is now Iwata, Shizuoka. Together with his father, he fought as a samurai in the Boshin War of the Meiji restoration.