May in O'Fallon averages a daily high of 24.6°C with nights around 12.2°C. Expect roughly 12.3 wet days totalling 127.9mm. Comfortable conditions for being outdoors all day.
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 May data.