Oak Grove, IL has a population of 562 and is the 18,837th largest city in the United States. The population density is 885 per sq mi which is 298% higher than the Illinois average and 876% higher than the national average. The median age in Oak Grove is 34 which is approximately 8% lower than the Illinois average of 37. In Oak Grove, 45% of the population over 15 years of age are married, 97% speak English and 1% speak Spanish. 63% of Oak Grove residents were born in Illinois, 31% were born out of state, 2% were born outside of the United States and 4% were foreign born.