O'Fallon, IL has a population of 29,078 and is the 1,471st largest city in the United States. The population density is 1,900 per sq mi which is 755% higher than the Illinois average and 1997% higher than the national average. The median age in O'Fallon is 37 which is approximately 2% lower than the Illinois average of 37. In O'Fallon, 61% of the population over 15 years of age are married, 94% speak English and 2% speak Spanish. 47% of O'Fallon residents were born in Illinois, 46% were born out of state, 3% were born outside of the United States and 4% were foreign born.