Ontario is the largest city in Malheur County, Oregon, United States. It lies along the Snake River at the Idaho border. The population was 11,366 at the 2010 census. The city is the largest community in the region of far eastern Oregon, also known as the Western Treasure Valley.Ontario is the principal city of the Ontario, OR-ID Micropolitan Statistical Area, which includes Malheur County in Oregon and Payette County in Idaho.Ontario is approximately halfway between Portland and Salt Lake City. It is the closest city to the Idaho border along Interstate 84. The city's slogan is "Where Oregon Begins".