Ontario is the largest city in Malheur County, Oregon, United States. It lies along the Snake River at the Idaho border. The population was 10,985 at the 2000 census, with an estimated population of 11,245 in 2006. 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".