- Oak Harbor (Population: 2,841)
- Oak Hill (Population: 1,685)
- Oakwood (Population: 9,215)
- Oberlin (Population: 8,195)
- Obetz (Population: 3,977)
- Olmsted Falls (Population: 7,962)
- Ontario (Population: 5,303)
- Orange (Population: 3,236)
- Oregon (Population: 19,355)
- Orrville (Population: 8,551)
- Orwell (Population: 1,519)
- Ottawa (Population: 4,367)
- Oxford (Population: 21,943)
