Ohio - Area of significance: Architecture
| Old District 10 Schoolhouse |
| Old Federal Building and Post Office |
| Old First Church |
| Old Funeral Palor |
| Old Gothic Barns |
| Old Governor's Mansion |
| Old Harbour Store |
| Old Hotel, The |
| Old Interurban Station |
| Old Jib's Corner |
| Old Lawrence County Jail |
| Old Log Post Office |
| Old Mahoning County Courthouse |
| Old McKinley High School |
| Old Meigs County Courthouse And Chester Academy |
| Old Mill |
| Old North End Historic District |
| Old Ohio Union |
| Old Peace Lutheran Church |
| Old Post Office And Federal Building |
| Old Railroad Station |
| Old River Road Historic District |
| Old South Church |
| Old St. Mary's Church, School and Rectory |
| Old Stone Church |
| Old St. Patrick's Church |
| Old Tippecanoe Main Street Historic District |
| Old Union School |
| Old Union School |
| Old US Post Office |
| Old Vermilion Banking Company Building |
| Old Vermilion Mill |
| Old Washington Historic District |
| Old Wayne County Jail |
| Old West End District |
| Old Wood County Jail |
| Olin, Arvin, House |
| Olive Branch High School |
| Olive Furnace |
| Oliver, John G., House |
| Ollie's Tavern |
| Olmsted Falls Depot |
| Olmsted Falls Historic District |
| Olney, Charles, House and Gallery |
| Omar Chapel |
| Onesto Hotel |
| Ontario Building |
| O. P. Boyer’s and Sons Funeral Home |
| Open Air School |
| Oppmann Terrace |
| Oregon Historic District |
| Orton Memorial Laboratory |
| Osborne School |
| O'Shaughnessy Dam and Bridge |
| Otis Hospital |
| Ottawa County Courthouse |
| Ottawa Waterworks Building |
| Otterbein Mausoleum |
| Ourant's School |
| Our House |
| Our Lady of Mercy High School |
| Our Lady of Mount Carmel Church |
| Outville Depot |
| Overholt House |
| Overlook Road Carriage House District |
| Overmyer Building, The |
| Overmyer-Waggoner-Roush Farm |
| Over-the-Rhine Historic District |
| Oxford Railroad Depot and Junction House |
| Ozmun, Isaac and Maria, Farmstead |
| Pabodie, Professor William, House |
| Pacific Curios Antiques |
| Packard-Doubler House |
| Packard, James Ward, House |
| Paige, David R., House |
| Painesville City Hall |
| Palace Hotel |
| Palace Theater |
| Palace Theatre Building |
| Palmer House |
| Palmyra Center Hotel |
| Panek Block |
| Pansy Methodist Church and School Historic District |
| Park Avenue Baptist Church |
| Park Building |
| Park Flats |
| Park, Jonathan, House |
| Parkside Apartments |
| Park Synagogue |
| Parmelee House |
| Pataskala Banking Company |
| Pataskala Elementary School |
| Pataskala Jail |
| Pataskala Presbyterian Church |
| Pataskala Town Hall |
| Pataskala United Methodist Church |
| Patterson, Mary A., Memorial |
| Patterson, Samuel N., House |
| Paulding County Carnegie Library |
| Paulding County Courthouse |
Data from United States Park Service and Wikidata
