There are 6 US Post Offices in Richland County serving a total of 16,327 residents equating to 2,721 residents per post office. It's estimated that approximately 26,060 packages are handled across the 6 county post offices annually. Richland County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 16,233. Its county seat is Olney."