13 cities
| City | County | 2025 Pop. ↓ |
|---|---|---|
| Oregon City | Clackamas County | 38,465 |
| Oak Grove | Clackamas County | 17,528 |
| Oatfield | Clackamas County | 12,600 |
| Ontario | Malheur County | 12,004 |
| Oak Hills | Washington County | 11,484 |
| Oakridge | Lane County | 3,145 |
| Odell | Hood River County | 2,729 |
| Oakland | Douglas County | 951 |
| Oregon Shores | Klamath County | 719 |
| O'Brien | Josephine County | 532 |
| Orient | Clackamas County | 502 |
| Oceanside | Tillamook County | 433 |
| Ochoco West | Crook County | 392 |