December in O'Fallon averages a daily high of 6.4°C with nights around -2.9°C. Expect roughly 9.1 wet days totalling 79.2mm. A hard month to visit for comfort alone.
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 December data.