There are 5 US Post Offices in Osceola County serving a total of 6,378 residents equating to 1,276 residents per post office. It's estimated that approximately 10,180 packages are handled across the 5 county post offices annually. Osceola County is a county located in the U.S. state of Iowa. As of the 2010 census, the population was 6,462. It is named for Seminole war chief Osceola. The county seat is Sibley, named for H. H. Sibley of Minnesota."