| Name | What | Region | Country | Lat | Long | Elev ft. | Pop est |
|---|---|---|---|---|---|---|---|
| Ohio Camp | city | Montana | United States | 48.83 | -111.85 | 3487 | |
| Oilmont | city | Montana | United States | 48.74 | -111.84 | 3471 | |
| Olanda (historical) | city | Montana | United States | 47.12 | -105.61 | 3349 | |
| Old Agency | city | Montana | United States | 47.33 | -114.30 | 2598 | |
| Old Banks Place | city | Montana | United States | 48.92 | -111.12 | 4265 | |
| Old Ford Place | city | Montana | United States | 45.90 | -107.46 | 3349 | |
| Old Hellgate Village Trailer Park | city | Montana | United States | 46.88 | -114.08 | 3349 | |
| Old Town | city | Montana | United States | 47.23 | -114.70 | 4396 | |
| Old Weiss and French Place | city | Montana | United States | 46.81 | -111.92 | 3661 | |
| Old Whitcomb Place | city | Montana | United States | 47.99 | -107.98 | 2509 | |
| Olive | city | Montana | United States | 45.55 | -105.53 | 3349 | |
| Oliverville | city | Montana | United States | 47.18 | -114.87 | 2808 | |
| Ollie | city | Montana | United States | 46.58 | -104.08 | 3349 | |
| Olney | city | Montana | United States | 48.55 | -114.58 | 3349 | |
| Olson Place | city | Montana | United States | 47.47 | -112.01 | 4189 | |
| Oneill (historical) | city | Montana | United States | 46.15 | -105.15 | 2598 | |
| Opheim | city | Montana | United States | 48.86 | -106.41 | 3349 | |
| Ophir (historical) | city | Montana | United States | 47.93 | -110.47 | 2598 | |
| Opportunity | city | Montana | United States | 46.11 | -112.83 | 5029 | |
| Orchard Hills Mobile Home Park | city | Montana | United States | 48.01 | -114.06 | 3349 | |
| Orchard Homes | city | Montana | United States | 46.86 | -114.05 | 3349 | |
| Orinoco | city | Montana | United States | 46.30 | -106.59 | 2509 | |
| Oscar (historical) | city | Montana | United States | 48.42 | -109.61 | 3349 | |
| Ossette (historical) | city | Montana | United States | 48.59 | -106.01 | 3349 | |
| Oswego | city | Montana | United States | 48.06 | -105.88 | 2063 | |
| Otter | city | Montana | United States | 45.21 | -106.20 | 3625 | |
| Outback Mobile Home Settlement | city | Montana | United States | 48.41 | -114.34 | 3349 | |
| Outlook | city | Montana | United States | 48.89 | -104.78 | 2509 | |
| Ovando | city | Montana | United States | 47.02 | -113.13 | 4189 | |
| Ozan | city | Montana | United States | 46.70 | -113.24 | 4189 |