Feature Name | Class | Elev. meters | County | Latitude DMS | Longitude DMS |
| Oak Hill Elementary School | School | 209 | Lake | 41 17 12 N | 087 25 28 W |
| Oak Knoll Golf Course | Locale | 210 | Lake | 41 25 01 N | 087 23 20 W |
| Oak Ridge Center Shopping Center | Locale | 201 | Lake | 41 29 07 N | 087 26 50 W |
| Oak Ridge Prairie County Park | Park | 194 | Lake | 41 30 36 N | 087 23 05 W |
| Oakland School (historical) | School | 211 | Lake | 41 15 13 N | 087 27 36 W |
| Old Fashion Missionary Baptist Church | Church | 182 | Lake | 41 33 52 N | 087 24 31 W |
| Old Path Church of Holiness | Church | 190 | Lake | 41 34 48 N | 087 21 43 W |
| Olive Grove Missionary Baptist Church | Church | 183 | Lake | 41 35 37 N | 087 22 55 W |
| Onehundredthirtyseventh Avenue Park | Park | 218 | Lake | 41 22 07 N | 087 25 23 W |
| Open Door Church of God in Christ | Church | 182 | Lake | 41 36 08 N | 087 21 15 W |
| Optimist Park | Park | 181 | Lake | 41 34 22 N | 087 27 50 W |
| Ora L Wildermuth Branch Gary Public Library | Building | 186 | Lake | 41 36 05 N | 087 16 04 W |
| Orchard Drive Elementary School | School | 183 | Lake | 41 34 30 N | 087 26 27 W |
| Orchard Grove | Populated Place | 209 | Lake | 41 17 23 N | 087 21 13 W |
| Orchard Grove Cemetery | Cemetery | 208 | Lake | 41 17 12 N | 087 20 40 W |
| Orchard Park | Park | 189 | Lake | 41 33 13 N | 087 26 27 W |
| Orchard Park | Park | 185 | Lake | 41 33 09 N | 087 29 24 W |
| Osborn | Populated Place | 183 | Lake | 41 35 33 N | 087 28 08 W |
| Our Lady of Consolation Church | Church | 213 | Lake | 41 28 03 N | 087 21 49 W |
| Our Lady of Grace Church | Church | 188 | Lake | 41 33 21 N | 087 27 18 W |
| Our Lady of Grace School | School | 187 | Lake | 41 33 22 N | 087 27 21 W |
| Our Lady of Guadalupe Roman Catholic Church | Church | 181 | Lake | 41 38 53 N | 087 26 47 W |
| Our Lady of Perpetual Help Church | Church | 183 | Lake | 41 34 59 N | 087 27 35 W |
| Our Lady of Perpetual Help Church (historical) | Church | 183 | Lake | 41 36 00 N | 087 20 30 W |
| Our Lady of Perpetual Help School | School | 183 | Lake | 41 35 03 N | 087 27 35 W |
| Our Savior Lutheran Church | Church | 199 | Lake | 41 32 12 N | 087 20 42 W |
| Palmer | Populated Place | 225 | Lake | 41 23 29 N | 087 14 20 W |
| Palmira Golf Course | Locale | 207 | Lake | 41 24 51 N | 087 29 04 W |
| Paul H Douglas Center or Environmental Education | School | 189 | Lake | 41 36 23 N | 087 16 06 W |
| Peace Baptist Church | Church | 182 | Lake | 41 35 25 N | 087 22 31 W |