- Oso is a census-designated place (CDP) in Snohomish County, Washington, United States. It is located to the west of Darrington, south of the North Fork of the Stillaguamish River and approximately 50 air miles (80 km) from Seattle. The population of Oso was 172 at the 2020 census. The area was the site of a large landslide in March 2014 that killed 43 people, the deadliest incident of its kind in U.S. history.
