[1] Oka Castle is located on the top of Tenjinyama, a long hill to the east of the urban center of Taketa, which is situated in a small basin in the southwestern part of Bungo Province.

It is at a crossroads of various roads crisscrossing the island of Kyushu and because of its geographical position, occupies an important strategic point controlling western Bungo Province.

Shiga Sadatomo, from one of the three major cadet branches of the Ōtomo clan, expanded the fortifications in 1334 and renamed it "Oka Castle".

but under Ōtomo Sōrin were defeated by the Shimazu clan at the Battle of Mimikawa and other locations in Hyuga Province in 1578.

During the Battle of Sekigahara in 1600, he remained loyal to the Eastern Army from the start of the campaign, and after the war, Tokugawa Ieyasu reconfirmed him in his existing holdings.Throughout the Edo period, the Nakagawa clan continued to rule Oka for 13 generations, without any transfer or reduction of territory.

It was also protected by yagura turrets, stone walls and combination gates, and in case of emergency could function as an independent castle.

The composer Rentarō Taki was inspired by the ruins to write the song Kōjō no Tsuki, which became famous by being included in the required curriculum for junior high school students in 1901 (although the lyrics were actually referring to Aizuwakamatsu Castle instead of Oka Castle.