会津戦争
Ōyama was born in Kagoshima to a "samurai" family of the Satsuma Domain. as a younger paternal cousin to Saigo Takamori. A protégé of Ōkubo Toshimichi, he worked to overthrow the Tokugawa Shogunate and thus played a major role in the Meiji Restoration. He served as the commander of the Detached First Brigade during the Boshin War. At the Battle of Aizu, Ōyama was the commander of the Satchōdo's field artillery positions on Mount Oda. During the course of the siege, he was wounded by an Aizu guerilla force under Sagawa Kanbei.