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.

[5] 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.

Oso is located along the Stillaguamish River North Fork, which forms a valley between several arms of the Cascade Range.

[9] The United States Census Bureau defines the Oso CDP as extending north from the river to the Skagit County border.

[9] The town was originally named Allen, but was renamed to Oso to avoid confusion with Allyn in Mason County.

[12] On Saturday, March 22, 2014, at 10:37 a.m. local time, a major mudflow occurred 4 miles (6.4 km) east of Oso, when a portion of an unstable hill collapsed, sending mud and debris across the North Fork of the Stillaguamish River, engulfing a rural neighborhood, and covering an area of approximately 1 square mile (2.6 km2).