29 cities
| City | County | 2025 Pop. ↓ |
|---|---|---|
| Oxford | Butler County | 22,469 |
| Oregon | Lucas County | 19,676 |
| Oakwood | Paulding County | 9,359 |
| Olmsted Falls | Cuyahoga County | 8,681 |
| Oberlin | Lorain County | 8,593 |
| Orrville | Wayne County | 8,437 |
| Obetz | Franklin County | 7,751 |
| Ontario | Richland County | 6,643 |
| Ottawa Hills | Lucas County | 4,800 |
| Ottawa | Putnam County | 4,412 |
| Orange | Cuyahoga County | 3,573 |
| Oakwood | Paulding County | 3,478 |
| Oak Harbor | Ottawa County | 2,782 |
| Orwell | Ashtabula County | 1,509 |
| Oak Hill | Jackson County | 1,402 |
| Ostrander | Delaware County | 1,259 |
| Ottoville | Putnam County | 967 |
| Owensville | Clermont County | 823 |
| Ohio City | Van Wert County | 644 |
| Oakwood | Paulding County | 536 |
| Osgood | Darke County | 298 |
| Orient | Pickaway County | 260 |
| Olde West Chester | Butler County | 253 |
| Oceola | Crawford County | 249 |
| Old Washington | Guernsey County | 221 |
| Orangeville | Trumbull County | 169 |
| Old Fort | Seneca County | 126 |
| Otway | Scioto County | 88 |
| Octa | Fayette County | 57 |