| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Thayer | 1 | 1,903 | 1,869 | 0.26% | 779 | 2 | City |
| Alton | 2 | 702 | 701 | 0% | 447 | 2 | City |
| Koshkonong | 3 | 198 | 196 | 0% | 1,106 | 0 | Town |
Thayer is the largest city in Oregon County, with a population of 1,903.
Koshkonong is the smallest city in Oregon County, with a population of 198.
There are 3 cities in Oregon County, Missouri.