O'Fallon, MO has a population of 84,018 and is the 406th largest city in the United States. The population density is 2,835 per sq mi which is 3181% higher than the Missouri average and 3029% higher than the national average. The median age in O'Fallon is 37 which is approximately 5% lower than the Missouri average of 38. In O'Fallon, 63% of the population over 15 years of age are married, 93% speak English and 2% speak Spanish. 70% of O'Fallon residents were born in Missouri, 25% were born out of state, 0% were born outside of the United States and 4% were foreign born.