Ono Masanao served as a bushō during the Sengoku period. He was a chief retainer of the Ii clan, kokujin, or provincial landowners, in Iinoya in the Inasa District of Tōtōmi Province.
His father, Ono Hyōgo-no-suke Shigemasa, served Ii Naohira. Masanao also served as a senior retainer of the Ii, becoming a chief retainer of Naohira’s grandson, Ii Naomori or Naomori’s uncle, Ii Naomitsu.
Naomori did not have a son so Naomitsu promised to send his own son, Kamenosuke (later known as Ii Naochika), as an adopted heir of Naomori. This, however, invited opposition from retainers of the Ii clan who were against the succession by Naochika to the headship of the clan. In particular, Naomitsu did not for some time get along well with Masanao, a chief retainer of the Ii family.
In 1541, the Ii were subordinate to the Imagawa clan who served as the lords of Suruga Province. The Kai-Takeda clan began to encroach on the territory of the Ii in Tōtōmi so, within the Ii family, Naohira had his sons, Naomitsu and Ii Naoyoshi, conduct military preparations. Masanao then slandered them by telling Imagawa Yoshimoto that Naomitsu and Naoyoshi were traitorous. On 12/23 of Tenbun 13 (1544), Naomitsu and Naoyoshi were summoned to Sunpu to explain themselves to Yoshimoto but were murdered instead. After returning from Sunpu to Iinoya, Masanao, following orders of Yoshimoto, attempted to kill Kamenosuke (Naochika) but, with the assistance of his chief retainer, Imamura Masazane, he fled to Shinano Province.
Masanao died in 1554 and was succeeded by his son, Ono Michiyoshi. Owing to the death of Masanao, Naomori called upon Naochika who had been residing in Shinano. In 1555, Naomori obtained permission from Imagawa Yoshimoto to bring back Naochika as his adopted heir.