8 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Sheldon | 1 | 5,624 | 5,503 | 0.36% | 1,171 | 5 | City |
| Hartley | 2 | 1,577 | 1,598 | -0.19% | 1,186 | 1 | City |
| Sanborn | 3 | 1,497 | 1,395 | 1.15% | 797 | 2 | City |
| Paullina | 4 | 980 | 976 | 0.1% | 1,049 | 1 | City |
| Primghar | 5 | 888 | 903 | -0.22% | 653 | 1 | City |
| Sutherland | 6 | 628 | 628 | 0% | 739 | 1 | City |
| Calumet | 7 | 143 | 146 | 0% | 538 | 0 | City |
| Archer | 8 | 116 | 116 | 0% | 1,365 | 0 | City |
Sheldon is the largest city in O'Brien County, with a population of 5,624.
Archer is the smallest city in O'Brien County, with a population of 116.
There are 8 cities in O'Brien County, Iowa.