Lifespan: 14xx to 15xx
Other Names: Ōi Muneaki, Takeda Kōunsai
Lord: Takeda Nobutora
Father: Ōi Nobukane
Siblings: Nobusato, Nobukore
Children: Ōi-no-kata (formal wife of Takeda Nobutora and mother of Takeda Shingen), Nobunari, Nobutsune, Mutō Nobutaka, Toramasa, Toranari, Mutō Tsuneaki, daughter (formal wife of Imai Nobumoto)
Ōi Nobusato served as a bushō during the Sengoku period. He was a kokujin, or provincial landowner, in Nishigōri in the western portion of Kai Province. Nobusato served as the lord of Ueno Castle (Tsubaki Castle).
The Ōi descended from the Takeda clan of the Kai-Genji and were founded by Ōi Nobuaki, the son of Takeda Nobutake, a bushō from the Nanbokuchō period. During this period in Kai, there was conflict among the kokujin in connection with internal discord pitting Takeda Nobutsuna (a sengoku daimyō, the seventeenth head of the Takeda clan, and the military governor of Kai) against Takeda Nobumasa (Nobutsuna’s father) and Aburakawa Nobuyoshi (Nobutsuna’s next younger brother).
In 1490, the Ōi clan confronted the Anayama clan in the Kawachi area of southern Kai, triggering a battle. To maintain their power, influential kokujin including the Anayama and others allied with the Imagawa clan of Suruga Province. The Ōi clan also affiliated with Imagawa Ujichika.
In an effort to expand his power through marital alliances, Nobusato had his daughters wed members of the Oyamada, the Hemi, and the Imai clans, eventually posing a challenge to the Takeda clan. In 1515, Nobusato was surrounded at Toda Castle by Takeda Nobutora, but, with the support of the Imagawa clan, was able to achieve a victory.
In 1516, the Imagawa clan invaded Kai, but, in 1517, withdrew from across the area including from Yoshida, and, later, reconciled with the Takeda. Similarly, Nobusato settled with Nobutora, offered his daughter, Ōi-no-kata (Zuiunin-dono), to become Nobutora’s formal wife while Nobusato then served him. Ōi-no-kata was the natural mother of Takeda Shingen. Nobusato, however, opposed orders from Nobutora for kokujin to move their residences to Kōfu. In 1520, Nobusato joined forces with his son-in-law, Imai Nobumoto, based in Hemi in the Koma District, and the Kurihara clan, to oppose Nobutora again, but was defeated in a battle in Imasuwa and, after surrendering, was compelled to retire. Records of a waka event held at the Ichiren Temple in Kōfu list the name of Ōi Nyūdō Muneaki so he is surmised to have been ordered by Nobutora to retire and enter the priesthood.
Ōi Nobunari inherited the headship of the clan, but Nobunari, and his son, Ōi Nobutame, died in succession so Ōi Nobutsune became the successor.
Nobusato was known to be a cultured individual. In 1506, a noble and poet named Asukai Masayasu gave him a compilation of waka, or classic Japanese poetry. After retiring, he frequently hosted waka events and was praised by another noble and poet named Reizei Tamekazu as a devotee of the art.