There are 4 US Post Offices in Ellis County serving a total of 4,169 residents equating to 1,042 residents per post office. It's estimated that approximately 6,654 packages are handled across the 4 county post offices annually. Ellis County is a county located on the western border of the U.S. state of Oklahoma. As of the 2010 census, the population was 4,151, making it the fifth-least populous county in Oklahoma. Its county seat is Arnett. The county was named for Albert H. Ellis, vice president of the 1906 Constitutional Convention.