| City | County | 2025 Pop. ↓ |
|---|---|---|
| Oxford | Butler County | 21,868 |
| Oregon | Lucas County | 19,565 |
| Oakwood | Paulding County | 9,253 |
| Olmsted Falls | Cuyahoga County | 8,541 |
| Orrville | Wayne County | 8,396 |
| Obetz | Franklin County | 7,935 |
| Oberlin | Lorain County | 7,123 |
| Ontario | Richland County | 6,647 |
| Ottawa Hills | Lucas County | 4,776 |
| Ottawa | Putnam County | 4,393 |
| Orange | Cuyahoga County | 3,541 |
| Oakwood | Paulding County | 3,421 |
| Oak Harbor | Ottawa County | 2,785 |
| Orwell | Ashtabula County | 1,506 |
| Oak Hill | Jackson County | 1,392 |
| Ostrander | Delaware County | 1,262 |
| Ottoville | Putnam County | 971 |
| Owensville | Clermont County | 818 |
| Ohio City | Van Wert County | 634 |
| Oakwood | Paulding County | 535 |
| Osgood | Darke County | 294 |
| Orient | Pickaway County | 260 |
| Olde West Chester | Butler County | 253 |
| Oceola | Crawford County | 249 |
| Old Washington | Guernsey County | 221 |
| Orangeville | Trumbull County | 167 |
| Old Fort | Seneca County | 126 |
| Otway | Scioto County | 88 |
| Octa | Fayette County | 58 |