City | 2024 Population |
|---|---|
| Oak Hills | 103 |
| Oakland | 1,516 |
| Oakland Acres | 177 |
| Oakville | 189 |
| Ocheyedan | 415 |
| Odebolt | 981 |
| Oelwein | 5,752 |
| Ogden | 1,948 |
| Okoboji | 727 |
| Olds | 185 |
| Olin | 654 |
| Ollie | 197 |
| Onawa | 2,786 |
| Onslow | 195 |
| Orange City | 6,437 |
| Orchard | 67 |
| Orient | 353 |
| Orleans | 504 |
| Osage | 3,504 |
| Osceola | 5,616 |
| Oskaloosa | 11,418 |
| Ossian | 776 |
| Osterdock | 42 |
| Otho | 406 |
| Oto | 68 |
| Otranto | 41 |
| Ottosen | 41 |
| Ottumwa | 25,184 |
| Owasa | 29 |
| Oxford | 708 |
| Oxford Junction | 423 |
| Oyens | 93 |