All - Architectural style - Neoclassical architecture
| Ocean Forest Country Club |
| Ocean Parkway Jewish Center |
| O'Connell, Clara T., School |
| Oconto County Courthouse |
| Odd Fellows Building |
| Odd Fellows Building |
| Odd Fellows Lodge |
| Odd Fellows Valley Lodge No. 189 Building |
| Odell Building |
| O'Donnell House |
| O'Donnell, I. D., House |
| Oeldorf Building/Wetherell's Jewelers |
| Oesterlein Machine Company-Fashion Frocks, Inc. Complex |
| Oglebay Hall |
| O'Grady, Mr. J. William de Coursey, House |
| Ohev Sholem Synagogue |
| Ohio National Bank |
| Ohio State Normal College At Kent |
| Ohio Theatre |
| Oil Springs High School Gymnasium |
| Oklahoma Publishing Company Building |
| Oklahoma State Capitol |
| Old Athens, Alabama Main Post Office |
| Old Bank of America Building |
| Old Cedar Baptist Church |
| Old Central Post Office |
| Old Clovis Post Office |
| Old Collier County Courthouse |
| Old Davidson County Courthouse |
| Old DeKalb County Courthouse |
| Oldenburg, Henry C., House |
| Old Farmers and Merchants State Bank |
| Old Federal Building-Federal Courthouse |
| Old Federal Reserve Bank |
| Old Hickory Methodist Church |
| Old Houston National Bank |
| Old Library Building |
| Old Louisiana Governor's Mansion |
| Old Morgantown Post Office |
| Old Nassau County Courthouse |
| Old Post Office |
| Old Post Office |
| Old Post Office And Federal Building |
| Old Roanoke County Courthouse |
| Old Rock House |
| Old Roman Catholic Cathedral |
| Old Sherman Public Library |
| Old Ship African Methodist Episcopal Zion Church |
| Old Sweet Springs |
| Old Ursuline Convent |
| Old U.S. Mint |
| Old US Post Office |
| Old US Post Office |
| Old US Post Office |
| Old U.S. Post Office and Federal Building |
| Old West Haven High School |
| Olean School #10 |
| Oliver, Dr. William Holt, House |
| Olmsted-Hixon-Albion Block |
| Olness, John, House |
| Olney Carnegie Library |
| Olsen, Lars and Christina, House |
| Oneida County Courthouse |
| Oneida Street Station |
| Ontario and Western Railroad Passenger Station |
| Ontonagon School |
| Opelousas City Hall |
| Opera House and I. O. O. F. Lodge |
| Orange Free Public Library |
| Orange High School |
| Orange Street Presbyterian Church |
| Orchestra Hall |
| Oregon Portland Cement Building |
| Orgain, Elbert S., House |
| Original Church of God |
| Orton Memorial Laboratory |
| Orton Plantation |
| Orville Beach Memorial Manual Training School |
| Osage County Courthouse |
| Osborne, Edmund B., House |
| Osborne Public Carnegie Library |
| Osgood Bradley Building |
| Oswego Public Carnegie Library |
| Otsego Hotel |
| Ottawa Library |
| Ottumwa Public Library |
| Our Lady of the Angels Academy |
| Overbrook |
| Overland Waterloo Company Building |
| Owatonna Free Public Library |
| Owen County Courthouse |
| Paca Street Firehouse |
| Packard, Milan and Margaret, House |
| Packard Motor Car Showroom and Storage Facility |
| Paddock, Augustus, House |
| Paddock, Colonel George H., House |
| Page Coal and Coke Company Store |
| Paia School |
| Paine Furniture Building |
| Paintsville Country Club |
Data from United States Park Service and Wikidata
