There are 3 US Post Offices in Decatur County serving a total of 2,810 residents equating to 937 residents per post office. It's estimated that approximately 4,485 packages are handled across the 3 county post offices annually. Decatur County (county code DC) is a county located in Northwest Kansas. As of the 2010 census, the county population was 2,961. Its county seat and most populous city is Oberlin. The county is named in honor of Commodore Stephen Decatur, Jr."