Born in 1529, Hidetsuna was the 18th head of the Oyama clan, descended from the Fujiwara-Hidesato line, and made Gion Castle (Oyama Castle) his stronghold. Sengoku-era Shimotsuke Province became a contested ground between the Hojo, Uesugi, and Satake clans, and Hidetsuna initially sought to maintain independence among these powers. In 1575, Hojo Ujiteru laid siege to Gion Castle, but Hidetsuna held out. The following year, 1576, when Ujiteru attacked again, Hidetsuna could no longer resist and surrendered the castle, fleeing with his heir Isechiyomaru (later Masatane) to take refuge with Satake Yoshishige in Hitachi. The castle was subsequently refortified by the Hojo as a base for campaigns into northern Kanto. After the Hojo's fall in the 1590 Odawara Campaign, Hidetsuna was permitted to return to part of his former territory and died in 1603.

Showed the resolve to withstand siege twice against the powerful Hojo, yet ultimately made the pragmatic decision to surrender the castle in order to preserve his clan's survival.

His surrender effectively ended the Oyama clan's role as an independent Sengoku daimyo house, though the family line survived. The Gion Castle ruins are today preserved as part of the "Oyama Clan Castle Ruins" National Historic Site.