8 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Sheldon | 1 | 5,604 | 5,503 | 0.36% | 1,167 | 5 | City |
| Hartley | 2 | 1,580 | 1,598 | -0.25% | 1,188 | 1 | City |
| Sanborn | 3 | 1,480 | 1,395 | 1.16% | 788 | 2 | City |
| Paullina | 4 | 979 | 976 | 0% | 1,048 | 1 | City |
| Primghar | 5 | 890 | 903 | -0.34% | 655 | 1 | City |
| Sutherland | 6 | 628 | 628 | 0% | 739 | 1 | City |
| Calumet | 7 | 143 | 146 | -0.69% | 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 5,604.
Archer is the smallest city in O'Brien County, with a population of 116.
There are 8 cities in O'Brien County, Iowa.