There are 9 US Post Offices in Jefferson County serving a total of 19,284 residents equating to 2,143 residents per post office. It's estimated that approximately 30,779 packages are handled across the 9 county post offices annually. Jefferson County (county code JF) is a county located in the U.S. state of Kansas. At the 2010 census, the county population was 19,126. Its county seat is Oskaloosa, and its most populous city is Valley Falls."