O'Brien, Oregon has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 4.1 °C (low) to 20.3 °C (high). Annual precipitation averages 1812.8 Millimeters. O'Brien is located in USDA Plant Hardiness Zone 8b. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Oct. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -0.1 °C | 9.4 °C | 291.34 mm |
| February | 0.2 °C | 12.4 °C | 242.06 mm |
| March | 1 °C | 14.9 °C | 252.48 mm |
| April | 2.6 °C | 18.1 °C | 155.7 mm |
| May | 5.4 °C | 23 °C | 67.82 mm |
| June | 7.9 °C | 27.4 °C | 24.38 mm |
| July | 10.4 °C | 33.2 °C | 5.08 mm |
| August | 9.5 °C | 32.9 °C | 5.33 mm |
| September | 6.9 °C | 29.2 °C | 24.89 mm |
| October | 3.6 °C | 21.6 °C | 122.43 mm |
| November | 1.9 °C | 12.7 °C | 255.02 mm |
| December | 0.2 °C | 8.4 °C | 366.27 mm |
| Annual | 4.1 °C | 20.3 °C | 1812.8 mm |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 75-100 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 | Moderate Drought |
| Köppen Climate Classification | Csb - Warm-Summer Mediterranean Climate |