Ooishi Yoshio was the chamberlain (karou) of the Ako han in Harima Province (now Hyogo Prefecture), Japan. He is known as the leader of the Forty-seven Ronin in their 1702 vendetta and thus the hero of the Chuushingura. He is often referred to by his title, Ooishi Kuranosuke.
He served Asano Naganori as the head chamberlain (hittougarou) 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, 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 Ako, Ooishi was in Ako 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 Ooishi would do as he had a reputation as an honorable and capable man.
He attempted to get the permission to re-establish the house of Asano but failed. He then began a careful plot to kill Lord Kira but to throw off suspicion spent time (and money) in the Geisha houses of Kyoto. He carried out his role well - and secretly coordinated the movements of the remaining loyal Asano samurai. Nearly two years passed before the attack was launched.
In January 1703, he, with 46 other Ako ronin, attacked Kira at his residence in the Honjo neighborhood of Edo. Kira was killed and decapitated. After Ooishi dispatched Terasaka Kichiemon to inform Asano's widow Youzeiin of the deed, he and the remaining 45 ronin went to Sengaku-ji in Shinagawa where Naganori was buried and there they were arrested.
He and the other ronin were sentenced to commit seppuku. Since it was an honorable sentence rather than merely decapitation, everyone accepted this sentence as an honor.
After death, Ooishi became famous as the example of ideal samurai behavior. His ultimate goal of reestablishment of the Asano clan was realized as the Tokugawa shogunate did restore some lands (about 1/10 of the original holdings) to the Asano clan.
Ooishi was in fact related to the Asano clan by blood, as several generations prior, an Ooishi man married a daughter of the Asano.