was a feudal domain under the Tokugawa shogunate of Edo period Japan, in what is now central à Âita Prefecture. It was centered around Oka Castle in what is now the city of Taketa, à Âita and was ruled by the tozama daimyà  Nakagawa clan for all of its history. It had the largest kokudaka of any domain in former Bungo Province. It was also sometimes referred to as
Oka Domain was founded by Nakagawa Hidenari, the son of Nakagawa Kiyohide, who had served Oda Nobunaga and Toyotomi Hideyoshi, and who had been awarded a 40,000 koku estate centered at Miki Castle in Harima Province. In 1594 Toyotomi Hideyoshi reassigned him to a new estate in Bungo Province with an increase to 66,000 koku. Subsequent surveys reassessed the official kokudaka to 70,000 koku. 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.
The third daimyà Â, Nakagawa Hisakiyo, invited the noted scholar Kumazawa Banzan to provide guidance on irrigation projects, increasing the wealth of domain and strengthening its military. The eighth daimyà Â, Nakagawa Hisasada, established the han school "Yugakukan", a martial arts school, "Keibukan", and a medical training school, "Hakusaikan".
During the Bakumatsu period, the samurai of Oka Domain strongly supported sonnà  jà Âi, however, the 12th daimyà Â, Nakagawa Hisaaki had been adopted from the Tà Âdà  clan, a strongly pro-Tokugawa clan, and expelled seven of the ringleaders from Oka Domain. Despite later appeals to change his position, Hisaaki remained neutral in the conflict, citing damage to the domain from large fires and windstorms and torrential rain, which placed the domain's finances in dire straits. Following the Meiji restoration in 1871, the domain became Oka Prefecture due to the abolition of the han system, and was later incorporated into à Âita Prefecture. The Nakagawa clan was elevated to the kazoku peerage with the title of count in 1884.
The noted late Edo Period painter Tanomura Chikuden, was from Oka Domain.
As with most domains in the han system, Oka Domain consisted of several discontinuous territories calculated to provide the assigned kokudaka, based on periodic cadastral surveys and projected agricultural yields.