There are 11 US Post Offices in Preble County serving a total of 41,519 residents equating to 3,774 residents per post office. It's estimated that approximately 66,269 packages are handled across the 11 county post offices annually. Preble County is a county located in the U.S. state of Ohio. As of the 2010 census, the population was 42,270. Its county seat is Eaton. The county was formed on February 15, 1808 from portions of Butler and Montgomery Counties. It is named for Edward Preble, a naval officer who fought in the American Revolutionary War and against the Barbary Pirates."