There are 25 US Post Offices in Marion County serving a total of 377,912 residents equating to 15,116 residents per post office. It's estimated that approximately 603,190 packages are handled across the 25 county post offices annually. Marion County is a county located in the U.S. state of Oregon. The population was 315,335 at the 2010 census, making it the fifth-most populous county in Oregon. The county seat is Salem, the state capital. The county was originally named the Champooick District, after Champoeg (earlier Champooick), a meeting place on the Willamette River. On September 3, 1849, the territorial legislature renamed it in honor of Francis Marion, a Continental Army general from South Carolina who served in the American Revolutionary War. Marion County is part of the Salem, OR Metropolitan Statistical Area, which is also included in the Portland-Vancouver-Salem, OR-WA Combined Statistical Area. It is located in the Willamette Valley."