8 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Sheldon | 1 | 5,370 | 5,500 | -0.48% | 1,118 | 4.80 | City |
| Hartley | 2 | 1,550 | 1,600 | -0.64% | 1,165 | 1.33 | City |
| Sanborn | 3 | 1,476 | 1,396 | 1.1% | 786 | 1.88 | City |
| Paullina | 4 | 962 | 976 | -0.31% | 1,030 | 0.93 | City |
| Primghar | 5 | 872 | 902 | -0.68% | 642 | 1.36 | City |
| Sutherland | 6 | 621 | 630 | -0.32% | 731 | 0.85 | City |
| Calumet | 7 | 138 | 145 | -0.72% | 519 | 0.27 | City |
| Archer | 8 | 112 | 116 | -0.88% | 1,318 | 0.09 | City |
Sheldon is the largest city in O'Brien County, with a population of 5,370.
Archer is the smallest city in O'Brien County.
There are 8 cities in O'Brien County, Iowa.