Oak Brook, IL has a population of 7,988 and is the 4,685th largest city in the United States. The population density is 1,003 per sq mi which is 351% higher than the Illinois average and 1007% higher than the national average. The median age in Oak Brook is 55 which is approximately 47% higher than the Illinois average of 37. In Oak Brook, 74% of the population over 15 years of age are married, 72% speak English and 2% speak Spanish. 60% of Oak Brook residents were born in Illinois, 12% were born out of state, 1% were born outside of the United States and 26% were foreign born.