- Wallowa County, Oregon has 3,145.9 square miles of land area and is the 9th largest county in Oregon by total area. Wallowa County, Oregon is bordered by Nez Perce County, Idaho, Union County, Oregon, Adams County, Idaho, Garfield County, Washington, Columbia County, Washington, Baker County, Oregon, Idaho County, Idaho, Umatilla County, Oregon, and Asotin County, Washington.