All - Architectural style - Greek Revival architecture
| Old Congregational Church |
| Old Courthouse |
| Old Courthouse |
| Old Courthouse, Warren County |
| Old Customshouse |
| Old Customshouse |
| Old Dawson Place |
| Old Denton |
| Old Dominion Bank Building |
| Old Downtown Harrisburg Commercial Historic District |
| Olde Store (John O'Ferrell Store) |
| Old Fort House |
| Old Fort House |
| Old Galveston Customhouse |
| Old Governor's Mansion |
| Old Homestead |
| Old Indian Meeting House |
| Old Kingsport Presbyterian Church |
| Old Knoxville City Hall |
| Old La Grange Schoolhouse |
| Old L & N Station |
| Old Mahoning County Courthouse |
| Old Main |
| Old Mill House |
| Old Morrison, Transylvania College |
| Old Nacogdoches University Building |
| Old Observatory |
| Old Orange County Courthouse |
| Old Pierce House |
| Old Plover Methodist Church |
| Old Presbyterian Church |
| Old Richmond County Courthouse |
| Old Rock School |
| Olds, Alonzo W., House |
| Old Seminary Building |
| Old Shipbuilder's Historic District |
| Old South Union Church |
| Old Spring Tavern |
| Old State Bank |
| Old State Capitol |
| Old State Capitol |
| Old Statehouse |
| Old State House |
| Old St. Mary's Church, School and Rectory |
| Old Stone House Library, The |
| Old Stone Tavern |
| Old St. Paul's Methodist Episcopal Church |
| Old Third District Courthouse |
| Old Town |
| Old Town Hall and School |
| Old Town Historic District |
| Old Town Historic District |
| Old Town House |
| Old Union School |
| Old Union School |
| Old U.S. Post Office and Courts Building |
| Old Wing Mission |
| Old Wythe Historic District |
| Oliver, Capt. John, House |
| Olive, Richard, House |
| Olivet Presbyterian Church |
| Onancock Historic District |
| Onderdonk, Horatio Gates, House |
| O'Neal, James, House |
| Oneida Lake Congregational Church |
| Ontario and Livingston Mutual Insurance Office |
| Oppenheim and St. Johnsville Union Society Church |
| Orange County Courthouse |
| Orange Grove |
| Orange Grove Plantation House |
| Orange Hall |
| Orange Street Historic District |
| Orchard House |
| Orient Baptist Church |
| Orlean Historic District |
| Orleans County Courthouse Historic District |
| Orna Villa |
| Orton Plantation |
| Ortonville Mill |
| Osbornedale |
| Osborn House |
| Osburn, Mrs., House |
| Osterville Baptist Church |
| Otey, William Madison, House |
| Otsdawa Baptist Church |
| Otterburn |
| Ottinger, Henry, House |
| Overby, L. B., House |
| Overmyer-Waggoner-Roush Farm |
| Owen Chapel Church of Christ |
| Owen-Harrison House |
| Owen-Primm House |
| Owens, James, Farm |
| Oxford Congregational Church and Cemetery |
| Oxford Historic District |
| Oxnard Public Library |
| Packard-Doubler House |
| Packer Farm and Barkersville Store |
| Paddock-Hubbard House |
| Page, H. P., House |
Data from United States Park Service and Wikidata
