| City | County | 2026 Pop. ↓ |
|---|---|---|
| Ossian | Wells County | 3,470 |
| Osceola | St. Joseph County | 2,587 |
| Oakland City | Gibson County | 2,269 |
| Orleans | Orange County | 2,067 |
| Osgood | Ripley County | 1,605 |
| Odon | Daviess County | 1,376 |
| Owensville | Gibson County | 1,331 |
| Oxford | Benton County | 1,188 |
| Ogden Dunes | Porter County | 1,160 |
| Oolitic | Lawrence County | 1,142 |
| Otterbein | Benton County | 1,139 |
| Oldenburg | Franklin County | 652 |
| Oaktown | Knox County | 575 |
| Otwell | Pike County | 429 |
| Orland | Steuben County | 390 |
| Otter Lake | Steuben County | 366 |
| Oswego | Kosciusko County | 350 |
| Orestes | Madison County | 342 |
| Oakville | Delaware County | 306 |
| Ontario | LaGrange County | 272 |
| Owensburg | Greene County | 191 |
| Ora | Starke County | 167 |
| Otisco | Clark County | 135 |
| Onward | Cass County | 76 |