Ōshū (奥州市, Ōshū-shi) is the second largest city in Iwate Prefecture, Japan with a population of 127,804.
The city was founded on February 20, 2006, as the result of a merger between the cities of Esashi and Mizusawa, the towns of Maesawa and Isawa and the village of Koromogawa from Isawa District. The modern city traces its origins to the foundation of Fort Isawa by Sakanoue no Tamuramaro in 802 in what is now Mizusawa Ward. Ōshū is famous for its Maesawa Beef, numerous festivals, historic temples and shrines and especially Fujiwara no Sato, a theme park and movie lot based on the exploits of the Northern Fujiwaras in the 12th century. Many famous people have claimed Ōshū as their home including Ichiro Ozawa, the long-time leader of the Democratic Party of Japan.
At 993.35 square kilometers, Ōshū is the second largest community in Iwate Prefecture in terms of land area. The city lies in a fertile plain stradling 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 Koromo River marks the southern border. Ishibuchi Dam creates a reservoir on the upper reaches of the Isawa River near Mt. Yakeishi-dake. However a new reservoir will be created behind Isawa Dam which will completely submerge the old reservoir. Construction is scheduled to be completed in 2013. There is some concern among local citizens that the weight of the water in the reservoir will trigger an earthquake which could cause the dam to burst.
The first inhabitants of this area were the prehistoric Jōmon people but little remains of their presence except arrowheads and pottery fragments. Isawa Ward is especially rich in Kofun Period remains. Next to Route 397 in Isawa Ward is the Tsunozuka-kofun Tumulus which dates from the 5th century.