August in O'Fallon averages a daily high of 30.5°C with nights around 18.3°C. Expect roughly 8.6 wet days totalling 92.3mm. Generally workable, with some weather to plan around.
Across the year, the best time to visit O'Fallon is September–October.
Based on ST PETERS 2 SE, MO US (10.7km), 1995–2024. 11 years of August data.