| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Sheldon | 1 | 5,592 | 5,499 | 0.3% | 1,164 | 5 | City |
| Hartley | 2 | 1,572 | 1,601 | -0.44% | 1,182 | 1 | City |
| Sanborn | 3 | 1,478 | 1,393 | 1.03% | 787 | 2 | City |
| Paullina | 4 | 968 | 976 | -0.21% | 1,036 | 1 | City |
| Primghar | 5 | 890 | 904 | -0.45% | 655 | 1 | City |
| Sutherland | 6 | 625 | 629 | -0.16% | 735 | 1 | City |
| Calumet | 7 | 143 | 147 | -0.69% | 538 | 0 | City |
| Archer | 8 | 115 | 117 | -0.86% | 1,353 | 0 | City |
Sheldon is the largest city in O'Brien County, with a population of 5,592.
Archer is the smallest city in O'Brien County, with a population of 115.
There are 8 cities in O'Brien County, Iowa.