Oshima Peninsula in the context of "Ōnuma Quasi-National Park"

The Northeastern Japan Arc, also Northeastern Honshū Arc, is an island arc on the Pacific Ring of Fire. The arc runs north to south along the Tōhoku region of Honshū, Japan. It is the result of the subduction of the Pacific plate underneath the Okhotsk plate at the Japan Trench. The southern end of the arc converges with the Southwestern Japan Arc and the Izu–Bonin–Mariana Arc at the Fossa Magna (ja) at the east end of the Itoigawa-Shizuoka Tectonic Line (ITIL). This is the geologic border between eastern and western Honshū. Mount Fuji is at the point where these three arcs meet. To the north, the Northeastern Japan arc extends through the Oshima Peninsula of Hokkaidō. The arc converges in a collision zone with the Sakhalin Island Arc and the Kuril Island Arc in the volcanic Ishikari Mountains of central Hokkaidō. This collision formed the Teshio and Yūbari Mountains.

The Ōu Mountains form the backbone of the volcanic part of the inner arc that run from Natsudomari Peninsula in Aomori Prefecture south to Mount Nikkō-Shirane in Tochigi and Gunma prefectures. The volcanic front consists of four north to south lines of Quaternary volcanoes and calderas, which extend the length of the range. It also includes the Quaternary volcanoes of southwestern Hokkaido. The Dewa Mountains and the Iide Mountains are non-volcanic uplift ranges that run parallel to the west of the Ōu Mountains.

Hiyama Subprefecture (檜山振興局, Hiyama-shinkō-kyoku) is a subprefecture of Hokkaido Prefecture, Japan located on the Oshima Peninsula on the Sea of Japan side. It includes Okushiri Island. It was established in 1897.

The Matsumae Domain (松前藩), a prominent domain during the Edo period, was situated in Matsumae, Matsumae Island (Ishijima), which is currently known as Matsumae Town, Matsumae District, Hokkaido, via Tsugaru District, Oshima Province. The clan's leader, also known as the lord of the domain, constructed Matsumae Fukuyama Castle in the same location and henceforth, the clan came to be referred to as the Fukuyama clan. In 1868, the castle was relocated to Tatejo in Assabu-cho, Hiyama-gun, within the territory, and was known as Tatehan during the Meiji period. Initially, the Matsumae clan's jurisdiction was limited to the Wajin territory on the Oshima Peninsula in southwest Hokkaido. However, they gradually expanded their control over the Ezo region, which is now part of Hokkaido, and transformed it into their domain. The clan's economic foundation relied on their exclusive trade with the Ainu people of Ezo, which set them apart from the agricultural-based governance principles of the Shogunate system. Towards the end of the Edo period, the shogunate often assumed control over Ezo.