There are 2 US Post Offices in Gilliam County serving a total of 1,799 residents equating to 900 residents per post office. It's estimated that approximately 2,871 packages are handled across the 2 county post offices annually. Gilliam County is a county located in the U.S. state of Oregon. As of the 2010 census, the population was 1,871, making it the third-least populous county in Oregon. The county seat is Condon. The county was established in 1885 and is named for Cornelius Gilliam, who commanded the forces of the provisional government of Oregon after the Whitman Massacre."