The Ōu Mountains are a mountain range in the Tōhoku region of Honshū, Japan. It is the longest range in Japan and stretches 500 km south from the Natsudomari Peninsula of Aomori Prefecture to the Nasu volcanoes at the northern boundary of the Kantō region. Though long, the range is only about 35 kilometres wide. The highest point in the range is Mount Iw…The Ōu Mountains are a mountain range in the Tōhoku region of Honshū, Japan. It is the longest range in Japan and stretches 500 km south from the Natsudomari Peninsula of Aomori Prefecture to the Nasu volcanoes at the northern boundary of the Kantō region. Though long, the range is only about 35 kilometres wide. The highest point in the range is Mount Iwate, 2,038 metres.