Shimazu Nariakira
Ōkubo was born on 26 September 1830 in Kagoshima, Satsuma Province (present-day Kagoshima Prefecture) to Ōkubo Juemon (1794-1863; also known as Toshio and Shirō), a low-ranking retainer of Shimazu Nariakira, the "daimyō" of the Satsuma Domain, later given a minor official position, and his wife Minayoshi Fuku (1803-1864), daughter of a physician. Although the Ōkubo family were not of high status, they were of distinguished origin, thought to descend from the noble Fujiwara clan. The eldest of five children, Ōkubo studied at the same local school as Saigō Takamori, who was three years older. In 1846, Ōkubo was given the position of aide to the Satsuma Domain's archivist.