Ōishi Yoshio (大石 良雄?, April 24, 1659 – March 20, 1703) was the chamberlain (karō) of the Akō Domain in Harima Province (now Hyōgo Prefecture), Japan (1679 - 1701). He is known as the leader of the Forty-seven Ronin in their 1702 vendetta and thus the hero of the Chūshingura. He is often referred to by his title, Ōishi Kuranosuke (大石内蔵助?).
He served Asano Naganori as the head chamberlain (hittōgarō (筆頭家老?)) for the Asano estate, supervising the daily running of the castle and the samurai. Due to the Tokugawa rules which required all the daimyos to spend every other year in Edo (now called Tokyo) the chamberlain was a very important man and the defacto ruler of the estate when the daimyo was away. Having attained this office at a rather young age, he is said to have had the implicit trust of his lord.
When Asano committed seppuku as punishment for his failed attempt to kill Kira Yoshinaka in Edo castle and the Tokugawa shogunate abolished the house of Asano in Akō, Ōishi was in Akō and managed all its administrative issues. He persuaded other samurai to vacate the castle to the agents of the Shogunate. During the next two years many people wondered what Oishi would do as he had a reputation as an honorable and capable man.