Oso is located at 48°16'20" North, 121°55'9" West (48.272281, -121.919235).
According to the United States Census Bureau, the city has a total area of 3.71 mi2 (9.61 km2). 3.62 mi2 (9.38 km2) of it is land and 0.09 mi2 (0.24 km2) of it is covered by water. The total area is 2.45% water.
Oso is located in the Pacific Standard Time (PST) timezone, GMT-08:00. It observes Daylight Saving Time (DST).