Feature Name | Class | Elev. meters | County | Latitude DMS | Longitude DMS |
| Olive-Orange Township School | School | 226 | Meigs | 39 10 08 N | 081 50 39 W |
| Olivedale Park | Park | 257 | Fairfield | 39 42 21 N | 082 37 18 W |
| Olivegreen Post Office (historical) | Post Office | 260 | Noble | 39 44 40 N | 081 36 54 W |
| Olivehill Elementary School | School | 273 | Montgomery | 39 45 46 N | 084 17 29 W |
| Oliver Cemetery | Cemetery | 337 | Jefferson | 40 15 52 N | 080 43 17 W |
| Oliver Hazard Perry Elementary School | School | 184 | Cuyahoga | 41 35 14 N | 081 33 07 W |
| Oliver House | Building | 182 | Lucas | 41 38 36 N | 083 32 17 W |
| Oliver Run | Stream | | Meigs | 39 06 41 N | 081 57 07 W |
| Oliver Run | Stream | | Monroe | 39 40 18 N | 080 57 53 W |
| Oliver School (historical) | School | 222 | Defiance | 41 21 17 N | 084 37 53 W |
| Oliver Tunnel | Tunnel | 301 | Jefferson | 40 17 28 N | 080 43 23 W |
| Oliveroot School (historical) | School | 375 | Morrow | 40 24 33 N | 082 46 08 W |
| Olivesburg | Populated Place | 328 | Richland | 40 53 09 N | 082 25 44 W |
| Olivesburg Post Office (historical) | Post Office | 327 | Richland | 40 53 10 N | 082 25 45 W |
| Olivet Baptist Church | Church | 209 | Cuyahoga | 41 29 35 N | 081 37 30 W |
| Olivet Baptist Church | Church | 207 | Cuyahoga | 41 28 51 N | 081 43 19 W |
| Olivet Baptist Church | Church | 256 | Hamilton | 39 11 25 N | 084 24 28 W |
| Olivet Cemetery | Cemetery | 333 | Perry | 39 51 23 N | 082 21 25 W |
| Olivet Church | Church | 199 | Lucas | 41 42 49 N | 083 40 53 W |
| Olivet Church of the Brethren | Church | 334 | Perry | 39 51 15 N | 082 21 45 W |
| Olivet School (historical) | School | 299 | Morgan | 39 33 48 N | 081 44 30 W |
| Olivet United Brethren in Christ Church | Church | 298 | Montgomery | 39 46 28 N | 084 26 39 W |
| Olivett | Populated Place | 383 | Belmont | 40 01 19 N | 081 08 16 W |
| Olivett Post Office | Post Office | 380 | Belmont | 40 01 20 N | 081 08 14 W |
| Oller Cemetery | Cemetery | 271 | Delaware | 40 14 00 N | 083 08 33 W |
| Ollins Corners | Populated Place | 276 | Ashtabula | 41 47 58 N | 080 40 09 W |
| Ollum Cemetery | Cemetery | 200 | Monroe | 39 43 33 N | 080 51 47 W |
| Olmsted | Populated Place | 229 | Cuyahoga | 41 24 25 N | 081 53 48 W |
| Olmsted Falls | Populated Place | 234 | Cuyahoga | 41 22 30 N | 081 54 29 W |
| Olmsted Falls City Hall | Building | 234 | Cuyahoga | 41 22 34 N | 081 54 03 W |
| Olmsted Falls High School | School | 238 | Cuyahoga | 41 22 14 N | 081 55 36 W |
| Olmsted Falls Middle School | School | 235 | Cuyahoga | 41 22 24 N | 081 54 39 W |
| Olmsted Plaza Shopping Center | Locale | 228 | Cuyahoga | 41 25 18 N | 081 54 10 W |
| Olmsted Post Office (historical) | Post Office | 229 | Cuyahoga | 41 24 25 N | 081 53 49 W |
| Olmsted Township Hall | Building | 237 | Cuyahoga | 41 22 57 N | 081 55 20 W |
| Olney Avenue Elementary School | School | 297 | Marion | 40 34 43 N | 083 08 16 W |
| Olney Cemetery | Cemetery | 209 | Morgan | 39 35 22 N | 081 42 22 W |
| Olney Elementary School | School | 187 | Wood | 41 36 25 N | 083 28 29 W |
| Olney Run | Stream | | Morgan | 39 35 18 N | 081 42 23 W |
| Olney School | School | 381 | Belmont | 39 59 04 N | 081 09 00 W |
| Olscamp Hall | Building | 208 | Wood | 41 22 40 N | 083 38 17 W |
| Olson Hall | Building | 360 | Portage | 41 08 56 N | 081 20 44 W |
| Olson Research Center | Building | 311 | Summit | 41 04 46 N | 081 30 48 W |
| Olszeski Town | Populated Place | 231 | Jefferson | 40 11 40 N | 080 47 19 W |
| Olympia (historical) | Populated Place | 289 | Fayette | 39 32 28 N | 083 22 40 W |
| Olympic Hills (subdivision) | Populated Place | 323 | Clark | 39 56 56 N | 083 47 17 W |
| Omar | Populated Place | 274 | Seneca | 41 07 27 N | 082 52 17 W |
| Omar Cemetery | Cemetery | 272 | Seneca | 41 07 08 N | 082 51 47 W |
| Omar Chapel | Church | 274 | Seneca | 41 07 06 N | 082 51 47 W |
| Omar Elkhattab Mosque | Church | 227 | Franklin | 40 01 28 N | 083 01 44 W |
| Omar Post Office (historical) | Post Office | 274 | Seneca | 41 07 25 N | 082 52 15 W |
| Omega | Populated Place | 181 | Pike | 39 09 06 N | 082 54 37 W |
| Omega Cemetery | Cemetery | 180 | Pike | 39 08 58 N | 082 54 55 W |
| Omega Church of God | Church | 269 | Mahoning | 41 04 16 N | 080 35 51 W |
| Omega Missionary Baptist Church | Church | 289 | Montgomery | 39 45 38 N | 084 16 58 W |
| Omega Post Office (historical) | Post Office | 180 | Pike | 39 09 05 N | 082 54 35 W |
| Omni Plaza Shopping Center | Locale | 335 | Summit | 41 11 17 N | 081 26 28 W |
| Onaway Elementary School | School | 288 | Cuyahoga | 41 28 31 N | 081 34 23 W |
| One Hundred Fifth Street Church of God in Christ | Church | 211 | Cuyahoga | 41 29 53 N | 081 36 55 W |
| One Lake Erie Center | Building | 181 | Lucas | 41 39 05 N | 083 32 18 W |
| One to One Fitness Center | Building | 216 | Cuyahoga | 41 30 10 N | 081 36 21 W |
| Oneida | Populated Place | 200 | Butler | 39 28 42 N | 084 23 25 W |
| Oneida | Populated Place | 307 | Carroll | 40 41 55 N | 081 08 56 W |
| Oneida Elementary School (historical) | School | 202 | Butler | 39 29 02 N | 084 23 42 W |
| Oneida Post Office (historical) | Post Office | 308 | Carroll | 40 41 55 N | 081 08 54 W |
| Oneness Apostolic Church | Church | 251 | Licking | 40 03 43 N | 082 23 41 W |
| Onion Creek | Stream | | Vinton | 39 15 48 N | 082 17 18 W |
| Onion Run | Stream | | Morgan | 39 36 13 N | 081 42 08 W |
| Ontario | Populated Place | 414 | Richland | 40 45 34 N | 082 35 25 W |
| Ontario Cemetery | Cemetery | 412 | Richland | 40 45 38 N | 082 38 26 W |
| Ontario Elementary School | School | 177 | Erie | 41 26 59 N | 082 41 35 W |
| Ontario High School | School | 423 | Richland | 40 45 53 N | 082 38 52 W |
| Ontario Junior High School | School | 415 | Richland | 40 45 39 N | 082 38 34 W |
| Ontario Lagoon | Lake | 176 | Erie | 41 25 30 N | 082 21 46 W |
| Ontario Post Office | Post Office | 415 | Richland | 40 45 35 N | 082 35 25 W |
| Ontario United Methodist Church | Church | 413 | Richland | 40 45 38 N | 082 38 31 W |
| Opdyke Park | Park | 240 | Williams | 41 33 24 N | 084 30 50 W |
| Open Air School | School | 226 | Franklin | 40 00 53 N | 083 00 53 W |
| Open Bible Christian Church | Church | 271 | Montgomery | 39 45 38 N | 084 07 51 W |
| Open Bible House of God | Church | 160 | Hamilton | 39 06 14 N | 084 29 49 W |
| Open Door Baptist Church | Church | 209 | Cuyahoga | 41 29 19 N | 081 37 47 W |
| Open Door Baptist Church | Church | 188 | Lucas | 41 38 49 N | 083 35 11 W |
| Open Door Baptist Church | Church | 183 | Washington | 39 24 42 N | 081 27 41 W |
| Open Door Chapel | Church | 306 | Logan | 40 24 23 N | 083 58 28 W |
| Open Door Chapel | Church | 266 | Trumbull | 41 14 07 N | 080 49 26 W |
| Open Door Christian School | School | 235 | Lorain | 41 22 22 N | 082 09 34 W |
| Open Door Church | Church | 250 | Allen | 40 43 52 N | 084 16 52 W |
| Open Door Tabernacle | Church | 178 | Sandusky | 41 21 14 N | 083 07 05 W |
| Opera (historical) | Populated Place | 296 | Muskingum | 39 54 34 N | 082 09 46 W |
| Opera Post Office (historical) | Post Office | 303 | Muskingum | 39 54 35 N | 082 09 45 W |
| Opossum Creek | Stream | | Monroe | 39 44 21 N | 080 51 07 W |
| Opossum Creek | Stream | | Monroe | 39 44 22 N | 080 51 07 W |
| Opossum Creek | Stream | | Montgomery | 39 40 52 N | 084 16 06 W |
| Opossum Creek Bar | Bar | 187 | Monroe | 39 44 21 N | 080 50 57 W |
| Opossum Creek Reserve | Park | 254 | Montgomery | 39 42 34 N | 084 16 09 W |
| Opossum Hollow | Valley | | Adams | 38 38 33 N | 083 39 21 W |
| Opossum Hollow | Valley | | Clermont | 38 50 19 N | 084 13 40 W |
| Opossum Hollow | Valley | | Clermont | 38 54 12 N | 084 14 27 W |
| Opossum Hollow | Valley | | Coshocton | 40 20 16 N | 082 04 26 W |
| Opossum Hollow | Valley | | Jefferson | 40 20 57 N | 080 44 53 W |
| Opossum Hollow | Valley | | Morgan | 39 44 13 N | 081 55 00 W |
| Opossum Hollow | Valley | | Ross | 39 11 45 N | 082 52 43 W |
| Opossum Hollow | Valley | | Scioto | 38 42 07 N | 083 05 34 W |
| Opossum Isle School (historical) | School | 395 | Ashland | 40 34 20 N | 082 18 57 W |
| Opossum Run | Stream | | Brown | 38 50 47 N | 083 55 19 W |
| Opossum Run | Stream | | Clermont | 38 50 30 N | 084 13 57 W |
| Opossum Run | Stream | | Coshocton | 40 10 31 N | 082 02 47 W |
| Opossum Run | Stream | | Gallia | 39 01 49 N | 082 21 15 W |
| Opossum Run | Stream | | Guernsey | 39 55 01 N | 081 28 46 W |
| Opossum Run | Stream | | Highland | 39 19 34 N | 083 23 10 W |