Ohio - Significance region: Local
| Orrville Downtown Historic District |
| Orton Memorial Laboratory |
| Osborn, Charles S., House |
| Osborne School |
| O'Shaughnessy Dam and Bridge |
| Otis Hospital |
| Ottawa County Courthouse |
| Ottawa Waterworks Building |
| Otterbein Mausoleum |
| Ourant's School |
| Our Lady of Mount Carmel Church |
| Outville Depot |
| Outville Hay and Grain Company Building |
| Overholt House |
| Overlook Road Carriage House District |
| Overmyer Building, The |
| Overmyer-Waggoner-Roush Farm |
| Oviatt, Orson Minot, House |
| Oxford Female Institute |
| 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 Theatre |
| Palace Theatre Building |
| Palmyra Center Hotel |
| Panek Block |
| Pansy Methodist Church and School Historic District |
| Park Avenue Baptist Church |
| Park Building |
| Parker Covered Bridge |
| Park Flats |
| Park, Jonathan, House |
| Parkside Apartments |
| Parmelee House |
| Partridge, Reuben L. 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 Downtown Historic District |
| Paul Hall, Muskingum College |
| Peairs Homestead |
| Pease, George, House |
| Pease Homestead |
| Pebblebrook Farm House and Gardens |
| Peck, Judge William V., House |
| Peck, L. W., House |
| Peeble's Corner Historic District |
| Pelton House |
| Pelton, Old, Hotel |
| Penfield, Louis A., House |
| Pennsylvania Depot |
| Pennsylvania Railway Station |
| Peoples Bank Company Building |
| Perin Village Site |
| Perrill-Goodman Farm House |
| Perry, Norman Dewey, House |
| Perry-Payne Building |
| Perrysburg Water Maintenance Building |
| Pershing, Christian, Barn |
| Petersburg Mill |
| Peters Cartridge Company |
| Peters-Kupferschmid House |
| Peters, Stevenson, House |
| Pfarr Log House |
| Pfleger Family Houses |
| Philo II Archeological District |
| Phoenix Block |
| Phoenix Building/Cincinnati Club |
| Phoenix Mills |
| Piatt, Abram S., House and Piatt, Donn S., House |
| Pickands, Jay M., House |
| Pickerington Carnegie Library |
| Pickerington Depot |
| Piketon Mounds |
| Pilgrim Congregational Church |
| Pilgrim Presbyterian Church |
| Pine Meer |
| Pinney, Dr. Eli, House |
| Pinney Road Log Cabin |
| Piqua High School |
| Pisgah Christian Church |
| Pitman, Ben, House |
| Pittsburg, Cincinnati, and St. Louis Depot |
| Pitzer, Anthony, Jr., House |
| Plainview Farm |
| Plaza Apartments |
| Plaza Hotel |
| Pleasant Hill |
| Pleasant Hill United Church of Christ |
Data from United States Park Service and Wikidata
