O'Brien, Texas has a Humid Subtropical Climate climate (Köppen classification: Cfa). Average annual temperatures range from 50.3 °F (low) to 77 °F (high). Annual precipitation averages 25 Inches. O'Brien is located in USDA Plant Hardiness Zone 8a. The growing season typically runs from the last frost around Mar. 21 - Mar. 31 through the first frost around Nov. 11 - Nov. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 29.2 °F | 56.1 °F | 1.09 in. |
| February | 32.0 °F | 60.2 °F | 1.25 in. |
| March | 40.3 °F | 68.8 °F | 1.94 in. |
| April | 48.3 °F | 77.6 °F | 1.86 in. |
| May | 59.0 °F | 85.7 °F | 3.28 in. |
| June | 67.9 °F | 92.7 °F | 3.31 in. |
| July | 71.8 °F | 96.7 °F | 1.85 in. |
| August | 70.4 °F | 96.0 °F | 2.27 in. |
| September | 62.7 °F | 88.2 °F | 2.95 in. |
| October | 51.4 °F | 78.3 °F | 2.29 in. |
| November | 39.5 °F | 66.2 °F | 1.72 in. |
| December | 31.1 °F | 57.3 °F | 1.23 in. |
| Annual | 50.3 °F | 77 °F | 25.04 in. |
| Average First Frost Date | Nov. 11 - Nov. 20 |
| Average Last Frost Date | Mar. 21 - Mar. 31 |
| Average Days With Frost | 50-75 Days |
| Plant Hardiness Zone | Zone 8a : 10°F to 15°F |
| Plant Hardiness Zone - 2012 | Zone 7b : 5°F to 10°F |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | Cfa - Humid Subtropical Climate |