Lifespan: Kyōroku 4 (1531) to 11/8 of Tenshō 9 (1581)
Lord: Hōjō Ujiyasu
Father: Ōta Suketaka
Mother: Jōshinin (daughter of Hōjō Ujitsuna)
Siblings: Kagesuke, Yasusuke, Terusuke
Wife: Hōshōin (natural daughter of Tōyama Tsunakage, adopted by Hōjō Ujiyasu)
Children: Komachiyo, Shigemasa
Ōta Yasusuke served as a bushō during the Sengoku and Azuchi-Momoyama periods.
Yasusuke was born as the lineal heir (or second son) of Ōta Suketaka. His mother, Jōshinin, was the daughter of Hōjō Ujitsuna. He received one of the characters in his name from his maternal uncle, Hōjō Ujiyasu, and adopted the name of Yasusuke. We later wed the adopted daughter of Ujiyasu (the natural daughter of Tōyama Tsunakage) named Hōshōin.
Around 1547, after the death of Suketaka, Yasusuke inherited the headship of the clan and became the chamberlain of Edo Castle. Yasusuke had landholdings with a value of 2,000 kan (with under one-half under the direct control of the shōgun). Among retainers of the Hōjō, this corresponded to a value of sixth rank, indicating that Ujiyasu attributed Yasusuke a relatively high level of status within the clan. Those kunishū, or provincial landowners, of high rank were noted in the register of landholdings with the title of “dono.” Although Yasusuke had landholdings of 931 kan, 384 mon, he did not have the “dono” title in the register and is therefore deemed to have become a retainer with less influence in the Edo area.
Yasusuke was known for his military valor. In 1554, when the Hōjō fought against the Kai-Takeda in Suruga Province, he repelled a battalion led Hara Toratane, a veteran retainer of the Takeda. Yasusuke, however, resented the lack of rewards shown for his contributions. According to one theory, he was dissatisfied that he could not become the lord of Edo Castle built by his great-grandfather, Ōta Dōkan. In 1562, he plotted via Ōta Sukemasa to abandon the Hōjō in favor of the Uesugi family. In sources regarding the Hōjō family, Sukemasa and the Satomi clam instigated this betrayal. These designs, however, failed, and, in the tenth month of the same year, he fled to Sukemasa for safety. In 1563, Ujiyasu captured Musashi-Matsuyama Castle and, riding this momentum, made plans to attack Sukemasa and Yasusuke. As a result, Uesugi Kenshin demanded the Satomi clan, as landlords in Awa, Kazusa and Shimōsa provinces, to rescue these two men who were among the few kokujin in Musashi aligned with the Uesugi. Satomi Yoshitaka, a sengoku daimyō and the fifth head of the Awa-Satomi clan, obliged Kenshin by dispatching his lineal heir, Satomi Yoshihiro, as the commander-in-chief of a large army to enter Kōnodai Castle.
Early in 1564, the allied forces of the Ōta and Satomi clashed with the Hōjō. In an event known as the Second Battle of Kōnodai, the allied forces retreated in a major defeat to the Hōjō. With protection from the Satomi, Yasusuke fled for safety to Awa Province. Conflict with the Hōjō persisted thereafter. In 1581, Yasusuke was swept up in a rebellion by Masaki Noritoki during a succession struggle in the Satomi clan. Owing to complicity with Noritoki, he and Noritoki either killed themselves at Ōtaki Castle or may have been killed by retainers. According to one theory, Yasusuke escaped with Sukemasa to the Satake clan, but the course of events in the aftermath of the battle are uncertain.
There are several theories concerning the origins of a consort cherished by Tokugawa Ieyasu named Eishōin. Either she was the natural daughter of Yasusuke and Hōshōin (the adopted daughter of Hōjō Tsunakage) or she was the natural daughter of Edo Shigemichi (a daimyō and the ninth head of the Hitachi-Edo clan) and adopted by Yasusuke in 1591 after Shigemichi was ousted by the Satake clan and fled. If the latter case is true, then this is inconsistent with the foregoing account that Yasusuke took his life in 1581. The Ōta clan, which held territory in the Edo period with a value of 50,000 koku, were relatives of Eishōin but the lineage of the Ōta at this point is unclear. The genealogy of the Ōta has many irregularities; and, in the era of Ōta Sukemune, there is a high likelihood of fabrications.
According to records of battles in the Kantō, Yasusuke stood over two meters in height and had a large frame so he was called a giant. He had a deep voice like the sound of thunder and was strong enough to easily lift a boulder that would require thirty average men to raise. A separate account gave the same example of his remarkable strength.