July in O'Fallon averages a daily high of 31.1°C with nights around 19.4°C. Expect roughly 9.2 wet days totalling 124.1mm. 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 July data.