Feature Name | Class | Elev. meters | County | Latitude DMS | Longitude DMS |
| Oto Post Office | Post Office | 404 | Woodbury | 42 17 12 N | 095 56 07 W |
| Otranto | Locale | 353 | Mitchell | 43 27 32 N | 092 59 31 W |
| Otranto | Locale | 350 | Mitchell | 43 29 37 N | 093 00 27 W |
| Otranto | Populated Place | 357 | Mitchell | 43 27 30 N | 092 59 09 W |
| Otranto Bridge | Bridge | 344 | Mitchell | 43 27 30 N | 092 58 54 W |
| Otranto Park | Park | 346 | Mitchell | 43 27 35 N | 092 58 52 W |
| Otranto Post Office (historical) | Post Office | 351 | Mitchell | 43 29 38 N | 093 00 21 W |
| Otranto Post Office (historical) | Post Office | 353 | Mitchell | 43 27 31 N | 092 59 31 W |
| Otsego Cemetery | Cemetery | 320 | Fayette | 42 38 48 N | 091 52 41 W |
| Ottawa | Populated Place | 330 | Clarke | 41 01 48 N | 093 35 28 W |
| Ottawa | Populated Place | 246 | Polk | 41 18 43 N | 093 30 08 W |
| Ottawa Cemetery | Cemetery | 323 | Clarke | 41 01 23 N | 093 35 25 W |
| Ottawa School | School | 330 | Clarke | 41 01 51 N | 093 34 37 W |
| Otter Bay | Bay | 155 | Des Moines | 40 50 54 N | 091 03 59 W |
| Otter Creek | Populated Place | 311 | Jackson | 42 14 26 N | 090 40 57 W |
| Otter Creek | Stream | | Buchanan | 42 29 59 N | 091 57 34 W |
| Otter Creek | Stream | | Crawford | 42 05 20 N | 095 19 16 W |
| Otter Creek | Stream | | Fayette | 42 57 25 N | 091 37 26 W |
| Otter Creek | Stream | | Franklin | 42 44 44 N | 093 06 07 W |
| Otter Creek | Stream | | Greene | 42 05 15 N | 094 37 21 W |
| Otter Creek | Stream | | Hardin | 42 20 55 N | 093 07 40 W |
| Otter Creek | Stream | | Jackson | 42 15 04 N | 090 44 23 W |
| Otter Creek | Stream | | Linn | 42 02 60 N | 091 45 17 W |
| Otter Creek | Stream | | Louisa | 41 07 00 N | 091 07 51 W |
| Otter Creek | Stream | | Lyon | 43 18 03 N | 096 03 28 W |
| Otter Creek | Stream | | Muscatine | 41 35 19 N | 091 03 49 W |
| Otter Creek | Stream | | Polk | 41 44 50 N | 093 34 38 W |
| Otter Creek | Stream | | Tama | 41 53 51 N | 092 22 02 W |
| Otter Creek | Stream | | Warren | 41 20 13 N | 093 29 43 W |
| Otter Creek | Stream | | Washington | 41 30 17 N | 091 30 28 W |
| Otter Creek | Stream | | Winneshiek | 43 16 07 N | 092 01 16 W |
| Otter Creek | Stream | | Wright | 42 45 34 N | 093 55 56 W |
| Otter Creek Bridge | Bridge | 302 | Buchanan | 42 38 08 N | 091 54 56 W |
| Otter Creek Bridge | Bridge | 311 | Fayette | 42 41 09 N | 091 56 50 W |
| Otter Creek Cemetery | Cemetery | 452 | Crawford | 42 10 30 N | 095 25 51 W |
| Otter Creek Community Center | Building | 212 | Louisa | 41 10 08 N | 091 16 60 W |
| Otter Creek Community Hall | Building | 274 | Dubuque | 42 17 50 N | 090 41 10 W |
| Otter Creek County Ditch | Canal | 178 | Louisa | 41 10 47 N | 091 14 37 W |
| Otter Creek County Park | Park | 298 | Warren | 41 14 42 N | 093 31 53 W |
| Otter Creek Golf Course | Locale | 282 | Polk | 41 45 50 N | 093 34 26 W |
| Otter Creek Lake | Reservoir | 283 | Tama | 42 02 32 N | 092 31 11 W |
| Otter Creek Marsh State Wildlife Refuge | Park | 241 | Tama | 41 56 24 N | 092 26 32 W |
| Otter Creek Nature Area | Park | 370 | Crawford | 42 07 32 N | 095 19 46 W |
| Otter Creek Park | Park | 283 | Polk | 41 45 51 N | 093 34 26 W |
| Otter Creek Wildlife Area | Park | 298 | Buchanan | 42 37 09 N | 091 54 44 W |
| Otter Creek Wildlife Area | Park | 342 | Wright | 42 45 50 N | 093 55 59 W |
| Otter Island | Island | 158 | Des Moines | 40 51 29 N | 091 03 37 W |
| Otter Island | Island | 162 | Louisa | 41 11 14 N | 091 03 37 W |
| Otter Island | Island | 165 | Louisa | 41 11 46 N | 091 04 09 W |
| Otter Slough | Stream | | Des Moines | 40 50 29 N | 091 04 23 W |
| Otter Valley Golf Course | Locale | 433 | Sioux | 43 11 28 N | 096 00 21 W |
| Otterbein Cemetery | Cemetery | 323 | Union | 40 59 57 N | 094 05 18 W |
| Otterbein Church | Church | 317 | Lucas | 40 56 43 N | 093 18 28 W |
| Otterbein South Cemetery | Cemetery | 293 | Appanoose | 40 42 14 N | 092 44 11 W |
| Otterbien Cemetery | Cemetery | 294 | Appanoose | 40 42 16 N | 092 44 17 W |
| Ottercreek Post Office (historical) | Post Office | 311 | Jackson | 42 14 26 N | 090 40 57 W |
| Otterman Cemetery | Cemetery | 291 | Dallas | 41 33 58 N | 093 59 02 W |
| Otterville | Populated Place | 282 | Buchanan | 42 30 31 N | 091 56 48 W |
| Otterville Bridge (historical) | Bridge | 276 | Buchanan | 42 30 43 N | 091 58 27 W |
| Otterville Bridge State Access | Locale | 276 | Buchanan | 42 30 58 N | 091 58 21 W |
| Otterville Cemetery | Cemetery | 282 | Buchanan | 42 30 57 N | 091 56 22 W |
| Otterville Post Office (historical) | Post Office | 282 | Buchanan | 42 30 31 N | 091 56 48 W |
| Otto B Laing Middle School | School | 368 | Kossuth | 43 04 03 N | 094 14 01 W |
| Otto Landing Strip | Airport | 181 | Louisa | 41 12 00 N | 091 14 24 W |
| Ottosen | Locale | 354 | Humboldt | 42 53 43 N | 094 22 48 W |
| Ottosen | Populated Place | 353 | Humboldt | 42 53 42 N | 094 22 48 W |
| Ottosen Marsh State Game Management Area | Park | 349 | Humboldt | 42 48 55 N | 094 20 51 W |
| Ottosen Post Office | Post Office | 353 | Humboldt | 42 53 40 N | 094 22 48 W |
| Ottosen Post Office | Post Office | 353 | Humboldt | 42 53 42 N | 094 22 48 W |
| Ottumwa | Locale | 195 | Wapello | 41 01 05 N | 092 24 56 W |
| Ottumwa | Populated Place | 205 | Wapello | 41 01 12 N | 092 24 41 W |
| Ottumwa Baptist Temple | Church | 199 | Wapello | 40 59 53 N | 092 25 37 W |
| Ottumwa Cemetery | Cemetery | 246 | Wapello | 41 02 05 N | 092 24 19 W |
| Ottumwa Christian School | School | 204 | Wapello | 41 01 32 N | 092 25 14 W |
| Ottumwa Church of Christ | Church | 198 | Wapello | 41 00 08 N | 092 25 36 W |
| Ottumwa City Fire Department | Building | 209 | Wapello | 41 01 20 N | 092 24 54 W |
| Ottumwa City Fire Department | Building | 197 | Wapello | 41 00 20 N | 092 25 20 W |
| Ottumwa City Hall | Building | 201 | Wapello | 41 01 05 N | 092 24 37 W |
| Ottumwa Coliseum | Building | 195 | Wapello | 41 00 47 N | 092 24 52 W |
| Ottumwa Community Church | Church | 206 | Wapello | 41 01 04 N | 092 24 33 W |
| Ottumwa Country Club | Locale | 254 | Wapello | 41 02 25 N | 092 24 11 W |
| Ottumwa Dermatology Clinic | Hospital | 213 | Wapello | 41 01 17 N | 092 24 44 W |
| Ottumwa Family Health Center | Hospital | 197 | Wapello | 41 01 05 N | 092 24 47 W |
| Ottumwa Family Practice Center | Hospital | 245 | Wapello | 41 01 40 N | 092 24 01 W |
| Ottumwa Foot and Ankle Clinic | Hospital | 254 | Wapello | 41 03 08 N | 092 24 31 W |
| Ottumwa Good Samaritan Center | Hospital | 227 | Wapello | 41 00 15 N | 092 27 01 W |
| Ottumwa High School | School | 221 | Wapello | 41 00 54 N | 092 24 20 W |
| Ottumwa Hospital Heliport | Airport | 254 | Wapello | 41 01 44 N | 092 23 27 W |
| Ottumwa Industrial Airport | Airport | 257 | Wapello | 41 06 24 N | 092 26 54 W |
| Ottumwa Junction | Populated Place | 222 | Wapello | 41 02 54 N | 092 26 23 W |
| Ottumwa Manor | Hospital | 249 | Wapello | 41 01 40 N | 092 23 29 W |
| Ottumwa Municipal Golf Course | Locale | 250 | Wapello | 41 03 45 N | 092 25 31 W |
| Ottumwa Obstetrics and Gynecology Center | Hospital | 255 | Wapello | 41 01 45 N | 092 23 23 W |
| Ottumwa Park Campground | Locale | 195 | Wapello | 41 00 52 N | 092 25 23 W |
| Ottumwa Pediatrics Center | Hospital | 255 | Wapello | 41 01 45 N | 092 23 23 W |
| Ottumwa Police Department | Building | 203 | Wapello | 41 01 08 N | 092 24 39 W |
| Ottumwa Post Office | Post Office | 200 | Wapello | 41 01 24 N | 092 25 09 W |
| Ottumwa Public Library | Building | 205 | Wapello | 41 01 10 N | 092 24 40 W |
| Ottumwa Regional Health Center | Hospital | 249 | Wapello | 41 01 40 N | 092 23 29 W |
| Ottumwa Union Depot | Building | 197 | Wapello | 41 01 08 N | 092 24 55 W |
| Our Lady of Angels Seminary | School | 180 | Clinton | 41 52 12 N | 090 11 04 W |
| Our Lady of Good Council Cemetery | Cemetery | 354 | Webster | 42 26 34 N | 094 18 17 W |
| Our Lady of Good Counsel Church | Church | 436 | Ida | 42 28 58 N | 095 32 10 W |
| Our Lady of Good Counsel Church | Church | 369 | Pocahontas | 42 35 07 N | 094 50 48 W |
| Our Lady of Good Counsel Church | Church | 351 | Webster | 42 26 30 N | 094 17 37 W |
| Our Lady of Good Counsel School | School | 370 | Pocahontas | 42 35 05 N | 094 50 48 W |
| Our Lady of Grace Catholic Church | Church | 335 | Cass | 41 14 07 N | 095 08 16 W |
| Our Lady of Grace Chapel | Church | 375 | Woodbury | 42 31 36 N | 096 25 33 W |
| Our Lady of Guadlupe Church | Church | 210 | Muscatine | 41 25 03 N | 091 03 25 W |
| Our Lady of Lourdes Catholic Church | Church | 171 | Scott | 41 31 19 N | 090 30 37 W |