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Ōshū is located in the south-central portion of Iwate Prefecture, bordered by the Akita Prefecture to the west. At 993.35 square kilometers, Ōshū is the second largest municipality in Iwate Prefecture in terms of land area. The city lies in a fertile plain straddling the Kitakami River and rises to the Ōu Mountains in the west and the Kitakami Mountains to the east. The city's highest point is Mt. Yakeishi-dake at 1,548 meters in the Ōu Mountains. The northern boundary is marked by the Isawa River while the Koromogawa River marks the southern border. Ishibuchi Dam creates a reservoir on the upper reaches of the Isawa River near Mt. Yakeishi-dake.