There are 6 US Post Offices in Washington County serving a total of 36,744 residents equating to 6,124 residents per post office. It's estimated that approximately 58,648 packages are handled across the 6 county post offices annually. Washington County is a county located in the northeastern part of the U.S. state of Oklahoma. As of the 2010 census, the population was 50,976. Its county seat is Bartlesville. Named for President George Washington, it is the smallest county in Oklahoma in total area, adjacent to the largest county in Oklahoma, Osage County. Washington County comprises the Bartlesville, OK Micropolitan Statistical Area, which is also included in the Tulsa-Muskogee-Bartlesville, OK Combined Statistical Area. It is located along the border with Kansas.