24 cities
| City | County | 2025 Pop. ↓ |
|---|---|---|
| Ossian | Wells County | 3,413 |
| Osceola | St. Joseph County | 2,597 |
| Oakland City | Gibson County | 2,243 |
| Orleans | Orange County | 2,047 |
| Osgood | Ripley County | 1,598 |
| Odon | Daviess County | 1,375 |
| Owensville | Gibson County | 1,333 |
| Ogden Dunes | Porter County | 1,167 |
| Oxford | Benton County | 1,167 |
| Oolitic | Lawrence County | 1,134 |
| Otterbein | Benton County | 1,121 |
| Oldenburg | Franklin County | 650 |
| Oaktown | Knox County | 586 |
| Ontario | 535 | |
| Otter Lake | 489 | |
| Oswego | 406 | |
| Orland | Steuben County | 397 |
| 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 | 77 |