Oregon is a city in and the county seat of Ogle County, Illinois, United States. The population was 3,721 in 2010. The land Oregon, Illinois …Oregon is a city in and the county seat of Ogle County, Illinois, United States. The population was 3,721 in 2010. The land Oregon, Illinois was founded on was previously held by the Potawatomi and Winnebago Indian tribes. In fact, later, settlers discovered that the area contained a large number of Indian mounds, most 10–12 feet in diameter.