Winters in O'Fallon are cold and unpredictable, with temperatures often dropping below freezing, but also experiencing mild days. Spring brings gradual warming, with temperatures increasing steadily from March to May, when highs can reach up to 32°C. Summer is warm, with July and August being the hottest months, with high temperatures averaging around 36°C. Autumn sees a return to cooler temperatures, with September and October having more moderate highs, before dropping again in November and December.