Pennsylvania - Criteria: C (architecture/engineering)
| Nutting Hall |
| Nyce Farm |
| Oakdale |
| Oakdale Public School |
| Oak Hall Historic District |
| Oak Hill Estate |
| Oakland Public School |
| Oaklands |
| Oakmont Country Club Historic District |
| Oak Park Historic District |
| O'Connor, J.S., American Rich Cut Glassware Factory |
| Octagon Stone Schoolhouse |
| O'Donel House and Farm |
| Ogden House |
| Ogontz Hall |
| Oil City Armory |
| Oil City Downtown Commercial Historic District |
| Oil City North Side Historic District |
| Oil City South Side Historic District |
| Okehocking Historic District |
| Old Allegheny Rows Historic District |
| Old Brown's Mill School |
| Old City Hall |
| Old City Historic District |
| Old Columbia-Wrightsville Bridge |
| Old Conemaugh Borough Historic District |
| Old Customshouse |
| Old Downtown Harrisburg Commercial Historic District |
| Old Dry Road |
| Old Federal Reserve Bank |
| Old Germantown Academy And Headmasters' Houses |
| Old Heidelberg Apartments |
| Old Homestead |
| Old Hoopes School |
| Old Indiana County Courthouse |
| Old Kennett Meetinghouse |
| Old Lehigh County Courthouse |
| Old Main |
| Old Main and Chemistry Building |
| Old Main at the Lutheran Home at Topton |
| Old Main, California State College |
| Old Main, Goethean Hall, and Diagnothian Hall |
| Old Mauch Chunk Historic District |
| Old Norriton Presbyterian Church |
| Old Pottstown Historic District |
| Old Rose Tree Tavern |
| Old Stone Arch Bridge |
| Old Swede's House |
| Old Town Historic District |
| Old Uptown Harrisburg Historic District |
| Old Waterworks |
| Old West, Dickinson College |
| Old White Mill |
| Oley Township Historic District |
| Oliver, David P., High School |
| Oller, Joseph J., House |
| Olney Elementary School |
| Olney High School |
| Oregon Mill Complex |
| Ormrod, George, House |
| Orris, Adam, House |
| Osterburg Covered Bridge |
| Osterling, Frederick J., Office and Studio |
| Ostheimer, Martha and Maurice, Estate |
| Overbrook Farms |
| Overbrook High School |
| Overbrook School |
| Oxford Historic District |
| Oxford Hotel |
| Oyer, Christian, Jr., House |
| Packard Motor Corporation Building |
| Packer, Asa, Mansion |
| Packer, Harry, Mansion |
| Packer, Isaac A., Farm |
| Packer Memorial Chapel |
| Packsaddle Bridge |
| Packwood House-American Hotel |
| Pagoda |
| Painter, William, Farm |
| Palmer, Amos, House |
| Paradise Valley Historic District |
| Pardee, Israel Platt, Mansion |
| Park Dinor |
| Parker Dam State Park Family Cabin District |
| Parker Dam State Park-Octagonal Lodge |
| Parker Dam State Park-Parker Dam District |
| Parkersville Friends Meetinghouse |
| Parkesburg National Bank |
| Parkhurst Memorial Presbyterian Church |
| Park Place School |
| Parkside Historic District |
| Parreco, James, House |
| Parr's Mill Covered Bridge No. 10 |
| Parsons-Taylor House |
| Passavant House |
| Passmore, Mansel, House |
| Paterson, John M., School |
| Patterson, Burd, House |
| Patterson Covered Bridge No. 112 |
| Patterson, Samuel, House |
Data from United States Park Service and Wikidata
