There are 23 US Post Offices in Clackamas County serving a total of 332,444 residents equating to 14,454 residents per post office. It's estimated that approximately 530,618 packages are handled across the 23 county post offices annually. Clackamas County /\u02c8kl\u00e6k\u0259m\u1d7bs/ is a county located in the U.S. state of Oregon. As of the 2010 census, the population was 375,992, making it the third-most populous county in Oregon. Its county seat is Oregon City. The county was named after the Native Americans living in the area, the Clackamas Indians, who were part of the Chinookan people. Clackamas County is included in the Portland-Vancouver-Hillsboro, OR-WA Metropolitan Statistical Area. It is located in the Willamette Valley."