- Oak Park (Population: 5,379)
- Oakland City (Population: 2,588)
- Odon (Population: 1,376)
- Ogden Dunes (Population: 1,313)
- Oolitic (Population: 1,152)
- Orleans (Population: 2,273)
- Osceola (Population: 1,859)
- Osgood (Population: 1,669)
- Ossian (Population: 2,943)
- Otterbein (Population: 1,312)
- Owensville (Population: 1,322)
- Oxford (Population: 1,271)
