Lifespan: Kyōroku 2 (1529) to 6/26 of Keichō 8 (1603)
Name Changes: Ujitomo → Ujihide → Ujitsuna → 孝哲 (Buddhist name)
Rank: bushō, sengoku daimyō
Title: Senior Assistant for Police and Security
Father: Oyama Takatomo
Siblings: Hidetsuna, Tomioka Hidetaka, Yūki Harutomo, sister (formal wife of Edo Shigemichi), Hamada Tomohide
Wife: [Formal] Adopted daughter of Hōjō Ujimasa (natural daughter of Narita Ujinaga)
Children: Hidehiro, Enomoto Takatsuna, Masatane, daughter (formal wife of Okamoto Zentetsu)
Oyama Hidetsuna served as a bushō and sengoku daimyō from the Sengoku to early Edo periods. He was the eighteenth head of the Oyama clan and the lord of Gion Castle in Shimotsuke Province.
In 1529, Hidetsuna was born as the eldest son of Oyama Takatomo, a sengoku daimyō and the seventeenth head of the Oyama clan. Initially, he received one of the characters from the name of his father and was known as Ujitomo, later changing to Ujihide. The character “uji” is surmised to have been conferred, in accordance with customary practice, by the Ashikaga family serving as the Koga kubō. Later, he adopted the name of Hidetsuna.
Around 1560, Hidetsuna inherited the headship of the clan. At this time in the Kantō, he found himself caught between two major powers: Uesugi Kenshin of Echigo Province who acquired the role of deputy shōgun of the Kantō from Uesugi Norimasa and backed Ashikaga Fujiuji (the lineal heir of Ashikaga Haruuji, the Koga kubō) and Hōjō Ujiyasu who backed Ashikaga Yoshiuji, the younger brother of Fujiuji (of a different mother).
In 1561, at the Siege of Odawara Castle, Hidetsuna joined Kenshin, participating in an assault on Odawara Castle defended by the Gohōjō clan. In 1563, he colluded with the Hōjō. In 1564, Hidetsuna was attacked by Kenshin at his base at Gion Castle and surrendered. The next year, he colluded again with the Hōjō. In a bid for the survival of the Oyama clan, he had relations with both camps. This caused frequent clashes with his younger brother, Yūki Harutomo, who, early on, sided with the Hōjō.
In 1575, Hōjō Ujiteru attacked Hidetsuna at his base at Gion Castle, but Hidetsuna was able to withstand the assault. At this time, Harutomo had already defected from the Hōjō clan and with the support of the neighboring Utsunomiya Hirotsuna and Satake Yoshishige, he proposed a settlement to Hōjō Ujimasa but was firmly rejected. In 1576, after incurring another attack, Hidetsune abandoned Gion Castle and, together with his lineal heir, Ise Chiyomaru (later known as Masatane), sought refuge in the territory of Yoshishige. Later, Gion Castle, in territory under the direct control of the Hōjō, became a base for attacks in northern Kantō.
Thereafter, Hidetsuna, with the assistance of Yoshishige, attempted to forcibly recover Gion Castle, but, in 1581, his lineal heir, Masatane, was killed in action so events did not unfold as planned.
On 5/18 of Tenshō 10 (1582), based on an edict by Oda Nobunaga prohibiting personal conflicts between daimyō or territorial disputes, through mediation by Takigawa Kazumasu (a senior retainer of Nobunaga), Hōjō Ujiteru returned Gion Castle to Hidetsuna. As a condition of the return, Hidetsuna had to serve the Hōjō clan and retainers of the Hōjō would be allowed to sojourn in Oyama. Consequently, Hidetsuna did not regain his former authority.
In 1590, during the Conquest of Odawara led by Toyotomi Hideyoshi, Hidetsuna was compelled to participate on the side of the Hōjō and was later removed from his position by Hideyoshi. The former territory of the Oyama was granted to his younger brother, Harutomo, whereupon Hidetsuna turned to Harutomo for support. His illegitimate eldest son, Oyama Hidehiro, became a senior retainer of the Yūki clan. In 1600, soon after the Battle of Sekigahara, Hidehiro died of illness so Hidetsuna did not follow the Yūki during their transfer to the Fukui domain and retired.
In 1603, Hidetsuna died of illness. This marked the end of the Oyama as a sengoku daimyō family. Oyama Hidetsune, the son of Hidehiro (Hidetsuna’s grandson), inherited the headship of the clan.