April in O'Fallon averages a daily high of 20.4°C with nights around 7.1°C. Expect roughly 10.8 wet days totalling 119mm. 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. 10 years of April data.