Onsong was one of the six post/garrisons (Korean: 육진; Hancha: 六鎭) established under the order of Sejong the Great of Joseon (1418–1450) to safeguard his people from the hostile Chinese Ming dynasty and Manchurian/Jurchen nomads living in Manchuria.

Onsong also contains the northernmost point in Korea, at 43°0'39″ N. Liangshui (Korean: 량수진; Hancha: 凉水鎮), in Tumen prefecture, is the closest Chinese town across the river.

It has a continental climate, and is the region of Korea with the longest winter, except the Kaema Plateau.

Ironically, however, it has the highest ratio of rice paddies to dry fields of any district in North Hamgyong Province.

[2] Onsŏng County is divided into 1 town ("Ŭp"), 10 workers' districts ("Rodongjagu") and 15 villages ("Ri"):