도사 번
Nakaoka was born in Tosa Domain, in what is now the village of Kitagawa, Kōchi Prefecture, as the son of a village headman. In 1861, he enrolled in the academy run by Takechi Hanpeita where he studied swordsmanship. He was one of the founding members of the "Tosa Kinno-tō", a paramilitary shishi organization created by Takeichi to support the "sonnō jōi" movement. After the coup d’etat of September 30, 1863, led to the suppression of the "sonnō Jōi" faction, he fled to Chōshū Domain together with pro-"Sonnō Jōi" nobles, including Sanjō Sanetomi. In 1864, he participated in a failed plot to assassinate Shimazu Hisamitsu, and fought alongside Chōshū forces during the Kinmon Incident and the Bombardment of Shimonoseki. Later that year, as a member of the "Kaientai" formed by Sakamoto Ryōma, he worked to bring about the Satsuma-Chōshū Alliance and to obtain the backing and support of Sanjō Sanetomi for the project. In March, 1867, he travelled with Sakamoto Ryōma to their native Tosa Domain to negotiate a similar alliance between Tosa and Satsuma. In June, he began negotiations to further expand the new alliance to include Chōshū as well as Hiroshima Domain, but the issue was rendered moot when "shōgun" Tokugawa Yoshinobu formally returned governing power to the Emperor. Realizing that civil war was now increasingly probable, Nakaoka returned to Tosa and established the "Rikuentai" militia. Rikuentai was modeled after the "Kiheitai" in Chōshū in July.