There are 19 US Post Offices in Johnson County serving a total of 398,227 residents equating to 20,959 residents per post office. It's estimated that approximately 635,615 packages are handled across the 19 county post offices annually. Johnson County (county code JO) is a county located in the U.S. state of Kansas. As of the 2010 census, the county population was 544,179, making it the most populous county in Kansas. Its county seat is Olathe, and its most populous city is Overland Park. In the mid-19th century, this was part of the Shawnee Reservation after their removal from east of the Mississippi River. The people were later forced to move to Indian Territory in present-day Ottawa County, Oklahoma."