O'Brien, Oregon has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 39.4 °F (low) to 68.5 °F (high). Annual precipitation averages 71.4 Inches. 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 | 31.9 °F | 48.9 °F | 11.47 in. |
| February | 32.4 °F | 54.4 °F | 9.53 in. |
| March | 33.8 °F | 58.9 °F | 9.94 in. |
| April | 36.6 °F | 64.5 °F | 6.13 in. |
| May | 41.8 °F | 73.4 °F | 2.67 in. |
| June | 46.2 °F | 81.4 °F | 0.96 in. |
| July | 50.7 °F | 91.7 °F | 0.2 in. |
| August | 49.1 °F | 91.2 °F | 0.21 in. |
| September | 44.5 °F | 84.5 °F | 0.98 in. |
| October | 38.4 °F | 70.9 °F | 4.82 in. |
| November | 35.4 °F | 54.8 °F | 10.04 in. |
| December | 32.3 °F | 47.1 °F | 14.42 in. |
| Annual | 39.4 °F | 68.5 °F | 71.37 in. |
| 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 : 15°F to 20°F |
| Plant Hardiness Zone - 2012 | Zone 8b : 15°F to 20°F |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | Csb - Warm-Summer Mediterranean Climate |