New York - Area of significance: Architecture
| Oak Street School |
| Oakwood Avenue Presbyterian Church |
| Oakwood Cemetery |
| Oakwood Cemetery |
| Oaterhoudt Stone House |
| Oblong Friends Meetinghouse |
| O'Brien General Store and Post Office |
| Ocean Parkway Jewish Center |
| Ocean Parkway Station (Dual System BRT) |
| Octagonal Schoolhouse |
| Octagon, The |
| Odd Fellows Hall |
| Odd Fellows Lodge and Temple |
| Odell House |
| O'Donohue, C. A., House |
| Offerman Building |
| Office Building and U.S. Light-House Depot Complex |
| Office of the Register |
| Ogdensburg Armory |
| Ogdensburg Harbor Lighthouse |
| Ohave Shalom Synagogue |
| Oheka |
| Ohio Street Methodist Episcopal Church Complex |
| O'Kane Market and O'Kane Building |
| Olbiston Flats |
| Old Allegany County Courthouse |
| Old Biology Hall |
| Old Brooklyn Fire Headquarters |
| Old Chappaqua Historic District |
| Old Colony Club |
| Old Courthouse Complex |
| Old Croton Dam, Site of |
| Old Episcopal Manse |
| Old Field Club and Farm |
| Old Field Point Light Station |
| Old First Church |
| Old First Reformed Church |
| Old Fort House |
| Old Grolier Club |
| Old Homer Village Historic District |
| Old House, The |
| Old Lutheran Parsonage |
| Old Madison County Courthouse |
| Old Merchant's House |
| Old Nassau County Courthouse |
| Old New York Evening Post Building |
| Old Parsonage |
| Old Post Office |
| Old Post Office |
| Old Quaker Meetinghouse |
| Old School Baptist Church of Halcottsville |
| Old Southeast Church |
| Old Southeast Town Hall |
| Old Stone Barracks |
| Old Stone Fort |
| Old Stone House Library, The |
| Old Stone Warehouse |
| Old St. Patrick's Cathedral Complex |
| Old St. Peter's Church |
| Old Town Green Historic District |
| Old Town Hall Historic District |
| Old Trees |
| Old Troy Hospital |
| Old Warren County Courthouse Complex |
| Old Westbury Gardens |
| Olean Armory |
| Olean Public Library |
| Olean School #10 |
| Olive and Hurley Old School Baptist Church |
| Oliver, Thomas, House |
| Olivet Chapel |
| Olmstead Street Historic District |
| Olmsted, Frederick Law, Sr., Farmhouse |
| Onderdonk, Horatio Gates, House |
| Onderdonk House |
| Oneida Armory |
| Oneida Community Limited Administration Building |
| Oneida Downtown Commercial Historic District |
| Oneida Lake Congregational Church |
| Oneonta Armory |
| Oneonta Downtown Historic District |
| Oneonta Theatre |
| Onesquethaw Valley Historic District |
| Onondaga County Savings Bank Building |
| Onondaga County War Memorial |
| Onondaga Highlands-Swaneola Heights Historic District |
| Ontario and Livingston Mutual Insurance Office |
| Ontario and Western Railroad Passenger Station |
| Onteora Park Historic District |
| Oppenheim and St. Johnsville Union Society Church |
| Oran Community Church |
| Oran District No. 22 Schoolhouse |
| Orange Mill Historic District |
| Orchard Shoal Light Station, Old |
| Ordway, Jones, House |
| Orient Historic District |
| Orient Point Light Station |
| Orleans County Courthouse Historic District |
| Ormonde |
| Ormsby-Laughlin Textile Companies Mill |
Data from United States Park Service and Wikidata
