Feature Name | Class | Elev. meters | County | Latitude DMS | Longitude DMS |
| Oakwoods Cemetery | Cemetery | 180 | Cook | 41 46 12 N | 087 36 00 W |
| Oakwoods Cemetery Chapel | Church | 180 | Cook | 41 46 18 N | 087 35 58 W |
| Oases School (historical) | School | 152 | Calhoun | 39 04 22 N | 090 36 45 W |
| Oasis Mobile Home Park | Populated Place | 200 | Cook | 42 00 50 N | 087 56 07 W |
| Oasis Trailer Park (subdivision) | Populated Place | 231 | McLean | 40 30 04 N | 089 00 22 W |
| Obed | Populated Place | 214 | Shelby | 39 33 55 N | 088 54 09 W |
| Obenchain School (historical) | School | 222 | Champaign | 40 16 08 N | 087 58 03 W |
| Oberle School (historical) | School | 187 | Henry | 41 33 02 N | 089 59 37 W |
| Oblate Fathers Novitiate | Church | 199 | Madison | 38 55 11 N | 090 14 04 W |
| Oblesby Post Office | Post Office | 193 | La Salle | 41 17 42 N | 089 03 46 W |
| Oblong | Populated Place | 156 | Crawford | 39 00 07 N | 087 54 32 W |
| Oblong Cemetery | Cemetery | 152 | Crawford | 39 00 04 N | 087 54 01 W |
| Oblong Elementary School | School | 156 | Crawford | 39 00 03 N | 087 54 53 W |
| Oblong High School | School | 160 | Crawford | 38 59 44 N | 087 54 34 W |
| Oblong Lake | Lake | 149 | Crawford | 39 00 16 N | 087 54 05 W |
| Ocean Wave School (historical) | School | 219 | Macon | 39 40 58 N | 088 54 22 W |
| Ochs School (historical) | School | 155 | Jasper | 38 55 21 N | 088 04 31 W |
| Oconee | Populated Place | 204 | Shelby | 39 17 08 N | 089 06 27 W |
| Oconee Mound Cemetery | Cemetery | 213 | Shelby | 39 18 07 N | 089 06 02 W |
| Ocoya | Populated Place | 204 | Livingston | 40 48 18 N | 088 41 02 W |
| Ocoya School (historical) | School | 205 | Livingston | 40 47 59 N | 088 40 52 W |
| Octagon School (historical) | School | 258 | Henry | 41 14 31 N | 090 00 24 W |
| Odd Fellows Cemetery | Cemetery | 135 | Johnson | 37 28 01 N | 088 46 28 W |
| Odd Fellows Cemetery | Cemetery | 130 | Madison | 38 44 19 N | 090 08 12 W |
| Odd Fellows Cemetery | Cemetery | 110 | Massac | 37 10 32 N | 088 43 41 W |
| Odd Fellows Cemetery | Cemetery | 197 | Whiteside | 41 47 01 N | 089 40 34 W |
| Odd Fellows Orphans Home of Illinois | Building | 182 | Logan | 40 08 21 N | 089 21 39 W |
| Oddfellow Cemetery | Cemetery | 139 | Franklin | 37 54 04 N | 088 54 19 W |
| Oddfellows Cemetery | Cemetery | 143 | Jefferson | 38 20 03 N | 089 01 19 W |
| Odell | Populated Place | 219 | Livingston | 41 00 13 N | 088 31 31 W |
| Odell Cemetery | Cemetery | 220 | Livingston | 41 00 40 N | 088 31 41 W |
| Odell Grade School | School | 217 | Livingston | 41 00 11 N | 088 31 15 W |
| Odell High School (historical) | School | 217 | Livingston | 41 00 11 N | 088 31 15 W |
| Odgen | Populated Place | 149 | Wabash | 38 27 07 N | 087 49 37 W |
| Odin | Populated Place | 161 | Marion | 38 37 02 N | 089 03 08 W |
| Odin High School | School | 160 | Marion | 38 36 56 N | 089 03 18 W |
| Odin Oil Field | Oilfield | 164 | Marion | 38 37 42 N | 089 01 43 W |
| Odum Spur | Locale | 137 | Franklin | 38 04 09 N | 088 45 14 W |
| Odumburg School (historical) | School | 165 | Williamson | 37 38 57 N | 088 49 57 W |
| Odyssey Golf Course | Locale | 215 | Cook | 41 31 51 N | 087 46 28 W |
| Oehrlein School of Cosmetology | School | 146 | Tazewell | 40 39 56 N | 089 33 26 W |
| Oesterle Library | Building | 219 | DuPage | 41 46 38 N | 088 08 34 W |
| Offenheiser Quarry | Mine | 245 | Stephenson | 42 16 34 N | 089 53 03 W |
| Offerman School (historical) | School | 190 | Will | 41 28 47 N | 088 06 54 W |
| Official Airline Guides Heliport | Airport | 200 | DuPage | 41 50 57 N | 087 56 00 W |
| Ogden | Populated Place | 204 | Champaign | 40 06 50 N | 087 57 22 W |
| Ogden - Cass Plaza Shopping Center | Locale | 226 | DuPage | 41 48 42 N | 087 58 25 W |
| Ogden Avenue Church of Christ | Church | 181 | Cook | 41 51 17 N | 087 42 56 W |
| Ogden Avenue Elementary School | School | 196 | Cook | 41 49 09 N | 087 52 36 W |
| Ogden Branch | Stream | | Pope | 37 34 38 N | 088 39 03 W |
| Ogden Cemetery | Cemetery | 228 | Edgar | 39 32 38 N | 087 41 43 W |
| Ogden Cemetery | Cemetery | 230 | Warren | 40 56 18 N | 090 31 58 W |
| Ogden Dam (historical) | Dam | 182 | Cook | 41 48 14 N | 087 48 09 W |
| Ogden Ditch (historical) | Canal | 182 | Cook | 41 49 00 N | 087 46 04 W |
| Ogden Elementary School | School | 204 | Champaign | 40 06 59 N | 087 57 29 W |
| Ogden Elementary School | School | 180 | Cook | 41 54 01 N | 087 37 45 W |
| Ogden Mounds | Locale | 140 | Fulton | 40 20 29 N | 090 07 03 W |
| Ogden Park | Park | 181 | Cook | 41 46 30 N | 087 39 23 W |
| Ogden Park Congregation of Jehovahs Witnesses | Church | 182 | Cook | 41 45 57 N | 087 38 41 W |
| Ogden School (historical) | School | 132 | Williamson | 37 40 41 N | 089 03 17 W |
| Ogden Slip | Gut | 175 | Cook | 41 53 25 N | 087 37 01 W |
| Ogilvie School (historical) | School | 223 | Ford | 40 58 55 N | 088 11 20 W |
| Ogle | Populated Place | 174 | Mercer | 41 06 26 N | 090 53 02 W |
| Ogle County | Civil | 260 | Ogle | 42 02 00 N | 089 19 00 W |
| Ogle County Courthouse | Building | 216 | Ogle | 42 00 50 N | 089 19 59 W |
| Ogle County Farm (historical) | Locale | 210 | Ogle | 41 59 33 N | 089 19 46 W |
| Ogle School (historical) | School | 223 | Stark | 41 02 50 N | 089 51 37 W |
| Ogles | Populated Place | 174 | St. Clair | 38 32 48 N | 090 03 17 W |
| Ogles Creek | Stream | | St. Clair | 38 38 49 N | 089 51 20 W |
| Ogles Station School (historical) | School | 171 | St. Clair | 38 32 35 N | 090 03 25 W |
| Oglesby | Populated Place | 192 | La Salle | 41 17 43 N | 089 03 34 W |
| Oglesby City Hall | Building | 192 | La Salle | 41 17 43 N | 089 03 44 W |
| Oglesby Hall | Building | 230 | Champaign | 40 05 56 N | 088 13 17 W |
| Oglesby Monument | Locale | 182 | Cook | 41 55 47 N | 087 38 14 W |
| Oglesby Public Library | Building | 192 | La Salle | 41 17 43 N | 089 03 44 W |
| Oglesby School | School | 182 | Cook | 41 45 17 N | 087 38 46 W |
| Oglesby Union Church | Church | 192 | La Salle | 41 17 43 N | 089 03 39 W |
| Ohio | Populated Place | 275 | Bureau | 41 33 27 N | 089 27 39 W |
| Ohio and Harding Park | Park | 184 | Cook | 41 53 31 N | 087 43 28 W |
| Ohio Cemetery | Cemetery | 271 | Bureau | 41 33 21 N | 089 27 33 W |
| Ohio Cemetery | Cemetery | 163 | Clark | 39 14 39 N | 087 42 08 W |
| Ohio Chapel | Church | 163 | Clark | 39 14 40 N | 087 42 11 W |
| Ohio Chapel | Church | 141 | Pulaski | 37 14 32 N | 088 58 28 W |
| Ohio Chapel Cemetery | Cemetery | 141 | Pulaski | 37 14 30 N | 088 58 26 W |
| Ohio Community Consolidated Grade School | School | 274 | Bureau | 41 33 28 N | 089 27 34 W |
| Ohio Community High School | School | 275 | Bureau | 41 33 30 N | 089 27 34 W |
| Ohio Grove (historical) | Woods | 261 | DeKalb | 41 57 07 N | 088 37 15 W |
| Ohio Grove Cemetery | Cemetery | 269 | DeKalb | 41 57 03 N | 088 39 39 W |
| Ohio Grove Church (historical) | Church | 213 | Mercer | 41 08 53 N | 090 40 32 W |
| Ohio Grove Grange | Locale | 265 | DeKalb | 41 56 09 N | 088 39 12 W |
| Ohio Grove School (historical) | School | 261 | DeKalb | 41 57 06 N | 088 37 16 W |
| Ohio Grove School (historical) | School | 206 | Mercer | 41 07 35 N | 090 41 44 W |
| Ohio Hall | Building | 226 | Kane | 42 03 40 N | 088 17 26 W |
| Ohio Park | Park | 185 | Cook | 41 53 30 N | 087 44 40 W |
| Ohio Public Library | Building | 277 | Bureau | 41 33 30 N | 089 27 42 W |
| Ohio River Recreation Area | Locale | 126 | Pope | 37 22 24 N | 088 29 11 W |
| Ohio River Recreation Area | Locale | 151 | Pope | 37 22 33 N | 088 28 55 W |
| Ohio School | School | 195 | Clark | 39 23 17 N | 087 41 43 W |
| Ohio School (historical) | School | 167 | Clark | 39 15 09 N | 087 42 11 W |
| Ohio School (historical) | School | 106 | Pulaski | 37 13 56 N | 089 00 52 W |
| Ohlman | Populated Place | 210 | Montgomery | 39 20 42 N | 089 13 07 W |
| Ohlman Cemetery | Cemetery | 231 | Christian | 39 21 23 N | 089 11 50 W |
| Ohlson Hall | Building | 179 | Cook | 41 58 31 N | 087 42 41 W |
| Ohlwine School (historical) | School | 128 | Randolph | 38 11 35 N | 089 58 46 W |
| OIC Vocational Institute | School | 223 | Winnebago | 42 16 34 N | 089 05 25 W |
| Oil Center | Populated Place | 154 | Crawford | 38 58 05 N | 087 49 58 W |
| Oil Grove | Populated Place | 169 | Crawford | 38 51 46 N | 087 38 30 W |
| Oilfield | Populated Place | 201 | Clark | 39 23 38 N | 087 59 19 W |
| Oink Acres Airport | Airport | 216 | Champaign | 40 09 38 N | 088 24 37 W |
| OK Flying Club Incorporated Airport | Airport | 201 | Montgomery | 39 11 41 N | 089 18 42 W |