| City | County | 2025 Pop. ↓ |
|---|---|---|
| Oregon City | Clackamas County | 37,072 |
| Oak Grove | Clackamas County | 17,762 |
| Oatfield | Clackamas County | 12,988 |
| Ontario | Malheur County | 11,949 |
| Oak Hills | Washington County | 11,892 |
| Odell | Hood River County | 3,190 |
| Oakridge | Lane County | 3,104 |
| Oakland | Douglas County | 961 |
| O'Brien | Josephine County | 648 |
| Orient | Clackamas County | 635 |
| Oceanside | Tillamook County | 500 |
| Oregon Shores | Klamath County | 485 |
| Ochoco West | Crook County | 292 |