Oso, Washington has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 5.2 °C (low) to 15.5 °C (high). Annual precipitation averages 1308.4 Millimeters. Oso is located in USDA Plant Hardiness Zone 8b. The growing season typically runs from the last frost around Apr. 1 - Apr. 10 through the first frost around Nov. 1 - Nov. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 0.6 °C | 7.3 °C | 164.34 mm |
| February | 0.6 °C | 9.1 °C | 118.11 mm |
| March | 1.9 °C | 11.4 °C | 137.92 mm |
| April | 4.1 °C | 14.8 °C | 112.27 mm |
| May | 7 °C | 18.7 °C | 76.45 mm |
| June | 9.4 °C | 20.8 °C | 66.8 mm |
| July | 11.1 °C | 24.6 °C | 29.46 mm |
| August | 11.1 °C | 25.2 °C | 30.99 mm |
| September | 8.8 °C | 21.8 °C | 64.52 mm |
| October | 5.3 °C | 15.4 °C | 135.64 mm |
| November | 2.5 °C | 10 °C | 194.31 mm |
| December | 0.4 °C | 6.7 °C | 177.55 mm |
| Annual | 5.2 °C | 15.5 °C | 1308.36 mm |
| Average First Frost Date | Nov. 1 - Nov. 10 |
| Average Last Frost Date | Apr. 1 - Apr. 10 |
| Average Days With Frost | 50-75 Days |
| Plant Hardiness Zone | Zone 8b : -9.4°C to -6.7°C |
| Plant Hardiness Zone - 2012 | Zone 8b : -9.4°C to -6.7°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Csb - Warm-Summer Mediterranean Climate |