Ontario is the largest city in Malheur County, Oregon, United States. It lies along the Snake River at the Idaho border. As of the 2000 census, the city population was 10,985. The 2006 estimate is 11,245 residents. The city is the largest community in the region of far eastern Oregon, also known as the Western Treasure Valley. Ontario apartments for rent comparison page shows most up to date rent information on Ontario apartments and houses. Rent information is obtained from multiple rental sources but we cannot guarantee their accuracy.