Ōkōchi Sadatsuna served as a bushō during the Sengoku period.  He was a retainer of the Kira clan, the lord of Hikuma Castle, and the local governor of the Hikuma manor in Tōtōmi Province.

From the Nanbokuchō period, the Ōkōchi clan were sent to serve as local governors of the Hikuma manor which was held by the Kira clan and gradually expanded their influence.  In the Muromachi period, during the Kanshō era (1460 to 1466), the names of Ōkōchi Hyōgo-no-suke Maie and Ōkōchi Bitchū-no-kami Nyūdō Michiaki appear in historical records and are deemed to be ancestors of Sadatsuna.  Sadatsuna had a younger brother named Komi Michitsuna but the names of siblings are not included in genealogies of the Ōkōchi clan who served as retainers of the Muromachi bakufu so the details of these relationships remain uncertain.

The Imagawa served as military governors of Tōtōmi for generations but accompanying the fall from power of Imagawa Sadayo in the early Muromachi period, the Shiba clan replaced them.  The Shiba appointed the Kai clan to serve as the deputy military governors to manage the territory.  Their efforts to govern, however, were impeded by Sadayo’s descendants, namely, the Tōtōmi-Imagawa clan in addition to the Yokochi and Katsumata clans (members of the hōkōshū, or military organ under the direct control of the shōgun), along with the Ōkōchi and Iinoo clans on behalf of the Kira clan.  Consequently, the Shiba were unable to fully control the province.  In the end, the heads of the Shiba clan died early one after another.  When Shiba Yoshitoshi came into conflict with Kai Jōchi at the Battle of Chōroku in 1458 to 1459, the band of retainers of the Shiba clan of Tōtōmi incited internal discord, further weakening the power of the clan.  Upon the outbreak of the Ōnin-Bunmei War, Imagawa Yoshitada of neighboring Suruga Province attacked, and with collaboration by Iinoo Nagatsura, the band of retainers of the Kira clan of Tōtōmi also triggered internal discord.  Thereafter, the Shiba and Imagawa engaged in direct conflict for control of Tōtōmi.

In the third month of 1501, Shiba Hiromoto and Shiba Yoshikatsu (younger brothers of Shiba Yoshihiro) deployed to Tōtōmi.  They joined forces with Ogasawara Sadatomo, the military governor of Shinano, to challenge the Imagawa army but were defeated by troops led by Asahina Yasuteru and Ise Sōzui.  At this time, Sadatsuna allied with Horie Shimotsuke-no-kami (who was on the side of the Shiba) and holed-up in Kuroyama Castle.  In the seventh month, the castle fell and Sadatsuna fled.  Inoo Katatsura was selected by the Imagawa as the local governor for the Hikuma manor.  Thereafter, based on their assessment of the situation in Tōtōmi, the Kira clan supported the Imagawa and cooperated with Imagawa Ujichika for the invasion of Mikawa Province.  Having been driven from his base, Sadatsuna joined with the Shiba and continued to resist the Imagawa to the end.

In the tenth month of 1511, Shiba Yoshitatsu deployed to Tōtōmi and from a base at Mitake Castle joined with Ii Naomune and Sadatsuna to mount an offensive.  Leading forces from Shinano, Mikawa, and Owari, Sadatsuna occupied Hikuma Castle by the fourth month of 1512.  He then attacked Horie Castle but failed and instead was attacked by Asahina Yasuhiro.  On 3/7 of Eishō 10 (1513), Sadatsuna surrendered and owing to an urgent appeal from the Kira clan his life was spared.  The main division led by Yoshitatsu was also defeated and withdrew to Owari while the Ii clan submitted to the Imagawa.

In 1515, Imagawa Ujichika intervened in an internal dispute of the Takeda clan and invaded Kai.  Taking advantage of this opening, in the third month of 1516, Sadatsuna raised arms again and occupied Hikuma Castle.  Shiba Yoshitatsu rushed from Owari to join the forces holed-up in Hikuma Castle.  In 1517, Ujichika responded by making peace with Takeda Nobutora and then deployed to Tōtōmi.  In the sixth month, he commenced an assault against Hikuma Castle.  Sadatsuna and the garrison stiffly resisted the besieging forces and did not waver when their supply lines were severed.  Nevertheless, after miners from the Umegashima gold mine severed the water supply to the castle well, the defenders were forced to capitulate.  On 8/19, the castle fell and Sadatsuna and his brother, Komi Michitsuna, took their own lives.

Shiba Yoshitatsu entered the priesthood and was sent to Owari while the Imagawa clan finally acquired control of Tōtōmi.