24 cities
| City | County | 2025 Pop. ↓ |
|---|---|---|
| Ossian | Wells County | 3,458 |
| Osceola | St. Joseph County | 2,604 |
| Oakland City | Gibson County | 2,264 |
| Orleans | Orange County | 2,071 |
| Osgood | Ripley County | 1,590 |
| Odon | Daviess County | 1,384 |
| Owensville | Gibson County | 1,331 |
| Oxford | Benton County | 1,187 |
| Ogden Dunes | Porter County | 1,169 |
| Otterbein | Benton County | 1,141 |
| Oolitic | Lawrence County | 1,135 |
| Oldenburg | Franklin County | 648 |
| Oaktown | Knox County | 580 |
| Ontario | 535 | |
| Otter Lake | 489 | |
| Oswego | 406 | |
| Orland | Steuben County | 393 |
| Otwell | Pike County | 352 |
| Orestes | Madison County | 343 |
| Oakville | Delaware County | 278 |
| Owensburg | Greene County | 188 |
| Ora | Starke County | 184 |
| Otisco | Clark County | 139 |
| Onward | Cass County | 75 |