Feature Name | Class | Elev. meters | County | Latitude DMS | Longitude DMS |
| Oriole Park | Park | 201 | Cook | 42 02 54 N | 087 48 53 W |
| Oriole Park | Park | 200 | Cook | 41 58 46 N | 087 48 56 W |
| Oriole Park School | School | 200 | Cook | 41 58 42 N | 087 48 46 W |
| Orion | Populated Place | 236 | Henry | 41 21 17 N | 090 22 53 W |
| Orion Church | Church | 235 | Henry | 41 19 49 N | 090 22 34 W |
| Orion High School | School | 238 | Henry | 41 21 13 N | 090 21 53 W |
| Orion Lutheran Cemetery | Cemetery | 236 | Henry | 41 21 26 N | 090 22 01 W |
| Orion Middle School | School | 239 | Henry | 41 21 07 N | 090 22 28 W |
| Orion Post Office | Post Office | 238 | Henry | 41 21 14 N | 090 22 54 W |
| Orion School (historical) | School | 205 | Fulton | 40 36 56 N | 089 58 03 W |
| Orland Center School | School | 215 | Cook | 41 36 56 N | 087 51 02 W |
| Orland Church (historical) | Church | 217 | Cook | 41 34 47 N | 087 49 24 W |
| Orland Consumer Square Shopping Center | Locale | 218 | Cook | 41 36 18 N | 087 47 46 W |
| Orland Greens Shopping Center | Locale | 219 | Cook | 41 37 01 N | 087 51 18 W |
| Orland Hills | Populated Place | 214 | Cook | 41 35 07 N | 087 50 35 W |
| Orland Junior High School | School | 220 | Cook | 41 37 10 N | 087 51 42 W |
| Orland Lake | Lake | 206 | Cook | 41 38 33 N | 087 52 03 W |
| Orland Park | Populated Place | 215 | Cook | 41 37 49 N | 087 51 14 W |
| Orland Park Cemetery | Cemetery | 204 | Cook | 41 37 23 N | 087 49 13 W |
| Orland Park Christian Reformed Church | Church | 221 | Cook | 41 36 54 N | 087 48 08 W |
| Orland Park Church | Church | 215 | Cook | 41 36 12 N | 087 53 40 W |
| Orland Park Elementary School | School | 209 | Cook | 41 37 51 N | 087 51 44 W |
| Orland Park Fire Station Number 1 | Building | 218 | Cook | 41 36 58 N | 087 51 24 W |
| Orland Park Fire Station Number 2 | Building | 213 | Cook | 41 36 59 N | 087 48 52 W |
| Orland Park Public Library | Building | 211 | Cook | 41 37 17 N | 087 52 01 W |
| Orland Park Shopping Plaza Shopping Center | Locale | 211 | Cook | 41 37 52 N | 087 51 20 W |
| Orland Park United Methodist Church | Church | 211 | Cook | 41 37 42 N | 087 51 46 W |
| Orland Park Village Center | Building | 221 | Cook | 41 37 14 N | 087 51 19 W |
| Orland Plaza Shopping Center | Locale | 210 | Cook | 41 36 09 N | 087 50 46 W |
| Orland Square Mall Shopping Center | Locale | 219 | Cook | 41 37 12 N | 087 50 54 W |
| Orland Square Village Shopping Center | Locale | 211 | Cook | 41 36 57 N | 087 50 32 W |
| Orland Towne Center Shopping Center | Locale | 212 | Cook | 41 36 01 N | 087 50 44 W |
| Orland Township Office | Building | 215 | Cook | 41 36 56 N | 087 50 56 W |
| Orland Tract Forest Preserve | Park | 221 | Cook | 41 34 32 N | 087 51 43 W |
| Orleans | Populated Place | 199 | Morgan | 39 43 25 N | 090 04 46 W |
| Orleans Court | Locale | 178 | Cook | 41 53 27 N | 087 38 10 W |
| Orleans Plaza | Building | 178 | Cook | 41 53 23 N | 087 38 15 W |
| Ormonde | Populated Place | 235 | Warren | 40 51 18 N | 090 36 47 W |
| Orpheum Theater (historical) | Building | 136 | Williamson | 37 43 49 N | 088 55 39 W |
| Orr Community Academy High School | School | 185 | Cook | 41 53 39 N | 087 43 37 W |
| Orr Park | Park | 185 | Cook | 41 53 40 N | 087 43 40 W |
| Orrington Elementary School | School | 183 | Cook | 42 03 56 N | 087 40 47 W |
| Ortend Cemetery | Cemetery | 277 | McHenry | 42 21 25 N | 088 20 00 W |
| Orth Cemetery | Cemetery | 263 | Boone | 42 19 37 N | 088 52 57 W |
| Orth School (historical) | School | 268 | Boone | 42 19 38 N | 088 53 14 W |
| Orthodox Cemetery | Cemetery | 251 | Henry | 41 15 22 N | 089 54 08 W |
| Orton Cemetery | Cemetery | 224 | Brown | 39 59 50 N | 090 54 30 W |
| Orton Landing Field | Airport | 196 | Greene | 39 25 23 N | 090 32 54 W |
| Orton School (historical) | School | 156 | Franklin | 37 56 10 N | 088 45 44 W |
| Orvis Cemetery | Cemetery | 264 | McHenry | 42 27 55 N | 088 12 30 W |
| Orwig RLA Airport | Airport | 169 | Tazewell | 40 24 40 N | 089 34 55 W |
| Osage Cemetery | Cemetery | 126 | Franklin | 37 53 20 N | 089 08 40 W |
| Osage Center School (historical) | School | 201 | La Salle | 41 03 33 N | 088 59 21 W |
| Osage Mine (historical) | Mine | 165 | La Salle | 41 19 42 N | 088 56 13 W |
| Osage Park | Park | 229 | Cook | 42 08 59 N | 088 01 20 W |
| Osage School (historical) | School | 195 | Montgomery | 39 07 48 N | 089 27 25 W |
| Osage Township Hall | Building | 201 | La Salle | 41 03 31 N | 088 59 20 W |
| Osage Youth Group Campground | Locale | 143 | Clinton | 38 40 15 N | 089 18 45 W |
| Osbernville | Populated Place | 185 | Christian | 39 45 40 N | 089 10 43 W |
| Osbernville Community Building | Building | 185 | Christian | 39 46 46 N | 089 09 40 W |
| Osborn | Populated Place | 178 | Rock Island | 41 31 21 N | 090 16 23 W |
| Osborn Park | Park | 218 | Cook | 42 01 30 N | 087 58 55 W |
| Osborne Park | Park | 187 | Cook | 41 42 12 N | 087 49 45 W |
| Osborne School (historical) | School | 168 | Jefferson | 38 24 09 N | 088 54 37 W |
| Osborne School (historical) | School | 126 | Perry | 38 00 09 N | 089 21 47 W |
| Osceola | Populated Place | 237 | Stark | 41 12 47 N | 089 46 47 W |
| Osceola Cemetery | Cemetery | 241 | Stark | 41 13 13 N | 089 46 34 W |
| Osceola Center | Locale | 226 | Stark | 41 11 54 N | 089 41 47 W |
| Osceola Grove Cemetery | Cemetery | 229 | Stark | 41 10 39 N | 089 45 55 W |
| Osceola Grove School (historical) | School | 228 | Stark | 41 10 41 N | 089 45 55 W |
| Osco | Populated Place | 235 | Henry | 41 20 52 N | 090 16 49 W |
| Osco Center School (historical) | School | 230 | Henry | 41 22 04 N | 090 15 41 W |
| Osco Community Church | Church | 234 | Henry | 41 20 47 N | 090 16 46 W |
| Osco Grange Hall (historical) | Building | 232 | Henry | 41 22 04 N | 090 13 19 W |
| Oskaloosa | Populated Place | 159 | Clay | 38 45 35 N | 088 38 56 W |
| Oskaloosa Cemetery | Cemetery | 158 | Clay | 38 45 47 N | 088 39 12 W |
| Oskaloosa Oil Field | Oilfield | 164 | Clay | 38 45 00 N | 088 37 26 W |
| Oskaloosa School (historical) | School | 162 | Clay | 38 46 02 N | 088 37 42 W |
| Osman | Populated Place | 228 | McLean | 40 17 45 N | 088 28 19 W |
| Osman Post Office (historical) | Post Office | 229 | McLean | 40 17 45 N | 088 28 17 W |
| Osmundson Cemetery | Cemetery | 198 | La Salle | 41 28 41 N | 088 38 43 W |
| Ospur | Populated Place | 221 | De Witt | 40 05 19 N | 088 57 19 W |
| Ossami Lake | Reservoir | 211 | Tazewell | 40 38 09 N | 089 28 06 W |
| Osteen School (historical) | School | 172 | Effingham | 39 01 28 N | 088 40 35 W |
| Ostend Post Office (historical) | Post Office | 253 | McHenry | 42 21 25 N | 088 21 18 W |
| Ostend School (historical) | School | 267 | McHenry | 42 21 22 N | 088 20 57 W |
| Oster-Oakview Middle School | School | 200 | Will | 41 31 11 N | 087 58 03 W |
| Ostermeir Pond | Reservoir | 179 | Christian | 39 32 12 N | 089 20 48 W |
| Ostermeir Pond Dam | Dam | 179 | Christian | 39 32 12 N | 089 20 48 W |
| Ostrander School (historical) | School | 208 | Kankakee | 41 14 33 N | 087 35 48 W |
| Oswego | Populated Place | 195 | Kendall | 41 40 58 N | 088 21 05 W |
| Oswego High School | School | 202 | Kendall | 41 40 57 N | 088 20 27 W |
| Oswego Post Office | Post Office | 197 | Kendall | 41 41 02 N | 088 21 02 W |
| Oswego Prairie Cemetery | Cemetery | 220 | Kendall | 41 40 39 N | 088 17 06 W |
| Oswego Prairie United Methodist Church | Church | 219 | Kendall | 41 41 36 N | 088 17 03 W |
| Oswego Presbyterian Church | Church | 196 | Kendall | 41 41 36 N | 088 20 47 W |
| Oswego Station (historical) | Locale | 201 | Kendall | 41 42 12 N | 088 22 05 W |
| Oswego Township Cemetery | Cemetery | 198 | Kendall | 41 40 39 N | 088 21 40 W |
| Oterlin Cemetery | Cemetery | 235 | Coles | 39 27 57 N | 088 01 08 W |
| Otgen Cemetery | Cemetery | 137 | Cass | 40 01 15 N | 090 23 55 W |
| Otis Elementary School | School | 181 | Cook | 41 53 30 N | 087 39 54 W |
| Ottawa | Populated Place | 147 | La Salle | 41 20 44 N | 088 50 33 W |
| Ottawa Airport | Airport | 186 | La Salle | 41 21 53 N | 088 51 33 W |
| Ottawa Avenue Cemetery | Cemetery | 146 | La Salle | 41 20 13 N | 088 51 59 W |
| Ottawa Boat Club | Locale | 146 | La Salle | 41 20 42 N | 088 50 28 W |
| Ottawa Canyon | Valley | | La Salle | 41 18 26 N | 088 56 43 W |
| Ottawa Park | Park | 189 | Will | 41 36 25 N | 088 11 49 W |
| Ottawa Picnic Area | Locale | 138 | Clinton | 38 40 23 N | 089 19 05 W |
| Ottawa Post Office | Post Office | 147 | La Salle | 41 20 44 N | 088 50 43 W |
| Ottawa Street United Methodist Church | Church | 165 | Will | 41 31 41 N | 088 04 58 W |