Mt. Takasaki Wild Monkey Park (高崎山自然動物園) is located in the west of the city, 20 min. by bus from JR Oita Sta. The area is the famous habitat of about 1,368 wild Japanese monkeys (as of January 2013) who live in a forest on the steep slope of Mt. Takasaki at a height of 628 m.
https://www.jnto.go.jp/eng/spot/zoo/mttakasaki.html