O'Brien, Texas has a Humid Subtropical Climate climate (Köppen classification: Cfa). Average annual temperatures range from 10.2 °C (low) to 25 °C (high). Annual precipitation averages 636 Millimeters. 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 | -1.6 °C | 13.4 °C | 27.69 mm |
| February | 0 °C | 15.7 °C | 31.75 mm |
| March | 4.6 °C | 20.4 °C | 49.28 mm |
| April | 9.1 °C | 25.3 °C | 47.24 mm |
| May | 15 °C | 29.8 °C | 83.31 mm |
| June | 19.9 °C | 33.7 °C | 84.07 mm |
| July | 22.1 °C | 35.9 °C | 46.99 mm |
| August | 21.3 °C | 35.6 °C | 57.66 mm |
| September | 17.1 °C | 31.2 °C | 74.93 mm |
| October | 10.8 °C | 25.7 °C | 58.17 mm |
| November | 4.2 °C | 19 °C | 43.69 mm |
| December | -0.5 °C | 14.1 °C | 31.24 mm |
| Annual | 10.2 °C | 25 °C | 636.02 mm |
| 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 : -12.2°C to -9.4°C |
| Plant Hardiness Zone - 2012 | Zone 7b : -15°C to -12.2°C |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | Cfa - Humid Subtropical Climate |