Founded in 1885, Gilliam County is one of 36 counties in the state of Oregon. Historically, Gilliam County was formed from Eastern third of Wasco County. The city of Condon is the county seat. With FIPS code of 21, Gilliam County has a population of 2,005 and area of 1,204 square miles (3,118 km2).
List of all cities in Gilliam County, OR alphabetically
| City/Town | Postal Code | Area Code |
| Arlington, OR | 97812 | 541 |
| 97861 | 541 | |
| Condon, OR | 97823 | 541 |
| Mikkalo, OR | 97861 | 541 |