Also known as: En ("Charming"), Lady Iwamura, Onao-no-kata

Lady Otsuya was daughter of Oda Nobusada and aunt of Oda Nobunaga. She was married to Tooyama Kagetou, a retainer of Oda Nobunaga.

When Tooyama died on Sept. 21, 1572 of illness without an heir, Nobunaga sent his fifth son Gobomaru (Oda Katsunaga), then seven, to be adopted by his aunt as heir to the Tooyama clan. Due to his young age, however, Lady Otsuya ruled the castle and the Tooyama domain.

Soon after, Takeda Shingen began his westward expansion and laid siege to the castle. Lady Otsuya negotiated for its surrender. The castle fell without bloodshed. As part of the terms, Gobomaru went to Takeda as hostage and Lady Otsuya married the Takeda-appointed lord of the castle, Akiyama Nobutomo, one of the Takeda Twenty-four Generals.

In 1575, after Takeda Shingen's death, Nobunaga laid siege to Iwamura Castle after defeating Takeda Katsuyori in the Battle of Nagashino. Akiyama Nobutomo and Lady Otsuya defended the castle against fierce assaults for six months, but acceded to Nobunaga's plea for peace and promise of safety. Nobunaga broke his promise and had both of them executed as traitors by hanging upside-down.

A local brewery makes a beer named "Lady Castellan" in her honor.