| City | County | 2026 Pop. ↓ |
|---|---|---|
| Oshkosh | Winnebago County | 67,507 |
| Oak Creek | Milwaukee County | 38,710 |
| Onalaska | La Crosse County | 20,550 |
| Oconomowoc | Waukesha County | 20,547 |
| Oregon | Dane County | 12,166 |
| Okauchee Lake | Waukesha County | 4,920 |
| Oconto | Oconto County | 4,646 |
| Omro | Winnebago County | 3,633 |
| Oostburg | Sheboygan County | 3,054 |
| Oconto Falls | Oconto County | 3,025 |
| Osceola | Polk County | 2,917 |
| Osseo | Trempealeau County | 1,845 |
| Orfordville | Rock County | 1,650 |
| Oakfield | Fond du Lac County | 1,068 |
| Owen | Clark County | 902 |
| Oconomowoc Lake | Dodge County | 569 |
| Oxford | Marquette County | 536 |
| Ontario | Vernon County | 527 |
| Oliver | Douglas County | 408 |
| Oakdale | Monroe County | 308 |
| Ogema | Price County | 276 |
| Ogdensburg | Waupaca County | 176 |
| Odanah | Ashland County | 145 |