- Oakland (Population: 1,487)
- Odebolt (Population: 1,153)
- Oelwein (Population: 6,692)
- Ogden (Population: 2,023)
- Onawa (Population: 3,091)
- Orange City (Population: 5,582)
- Osage (Population: 3,451)
- Osceola (Population: 4,659)
- Oskaloosa (Population: 10,938)
- Ottumwa (Population: 24,998)
