도사 번
Ōe was born in Hata District, Tosa Domain, what is now Ōtsuki, Kōchi. He grew up in Nagasaki, but towards the end of the Bakumatsu period returned to his native Tosa to join the "Rikuentai", a paramilitary group of Tosa "samurai" supporting the "Sonnō jōi" movement and dedicated to the overthrow of the Tokugawa shogunate. His fellow members included Sakamoto Ryōma, Nakaoka Shintarō and Mutsu Munemitsu. In 1868, he participated in an attack against the foreign consulates in Kobe.