There are 5 US Post Offices in Jefferson County serving a total of 25,694 residents equating to 5,139 residents per post office. It's estimated that approximately 41,010 packages are handled across the 5 county post offices annually. Jefferson County is a county located in the U.S. state of Oregon. As of the 2010 census, the population was 21,720. The county seat is Madras. The county is named after Mount Jefferson."