He was born in Satsuma, now Kagoshima prefecture as the eldest of five children. He studied at the same local school with Saigo Takamori, who was three years older. He became one of the mikado's principal ministers, and in the Satsuma troubles which followed he was the chief opponent of Saigo Takamori. But the suppression of the Satsuma rebellion brought upon him the personal revenge of Saigo's sympathizers, and in the spring of 1878 he was assassinated by six clansmen. Okubo was one of the leading men of his day, and in 1872 was one of the Japanese mission which was sent round the world to get ideas for organizing the new régime.
See also: Meiji Restoration