Maryland - Criteria: C (architecture/engineering)
| Ocean Hall |
| Octorara Farm |
| Odd Fellows Hall |
| Odd Fellows Lodge |
| O'Dea House |
| Old Bloomfield |
| Old Chiswell Place |
| Old City Hall and Engine House |
| Old Forge Farm |
| Old Goucher College Buildings |
| Old Hamilton Library |
| Old Inn, The |
| Old Lock Pump House, Chesapeake and Delaware Canal |
| Old Mill Road Bridge |
| Old Pine Street Station |
| Old Roman Catholic Cathedral |
| Old Salem Church and Cemetery |
| Old Town College Park |
| Old Town Friends' Meetinghouse |
| Old Town Savings Bank |
| Old Washington County Library |
| Old West Baltimore Historic District |
| Old Wye Church |
| OLIVER'S GIFT (log canoe) |
| Olney |
| One Charles Center |
| Orchard Street United Methodist Church |
| Orem's Delight |
| Orendorff, John, Farm |
| Otterbein Church |
| Otwell |
| Owens, James, Farm |
| Owensville Historic District |
| Oxford Historic District |
| Oxon Cove Park |
| Ozmon, Capt. John H., Store |
| Paca House and Garden |
| Paca Street Firehouse |
| Panther's Den |
| Paradise Manor |
| Park Circle Historic District |
| Parkhurst |
| Parkton Hotel |
| Pascault Row |
| PATRICIA (log canoe) |
| Patterson Archeological and Historic District |
| Patterson Park-Highlandtown Historic District |
| Patterson Viaduct Ruins |
| Peace Cross |
| Pearre-Metcalfe House |
| Pemberton Hall |
| Pennington Cottage |
| Pennsylvania Station |
| Pennterra |
| Perkins Square Gazebo |
| Perry-Cooper House |
| Perry Hall |
| PERSISTENCE (log canoe) |
| Phoenix Mill Farm |
| Pigtown Historic District |
| Pikesville Armory |
| Pine Street Neighborhood Historic District |
| Pipe Creek Friends Meetinghouse |
| Piper House |
| Pleasant Hill |
| Pleasant Hills |
| Pleasant Prospect |
| Plinlimmon Farm |
| Plumb Grove |
| Pocomoke City Historic District |
| Poffenberger Road Bridge |
| Point No Point Light Station |
| Point of Rocks Railroad Station |
| Polish Home Hall |
| Polychrome Historic District |
| Pomfret Plantation |
| Poole and Hunt Company Buildings |
| Poole, Nathan Dickerson, House |
| Pooles Island Lighthouse |
| Poolesville Historic District |
| Poplar Hill |
| Poplar Hill Mansion |
| Poppleton Fire Station |
| Port Deposit |
| Portland Manor |
| Porto Bello |
| Port Tobacco Historic District |
| Potomac-Broadway Historic District |
| Potter Hall |
| Pratt Street Power Plant |
| Presbury Meetinghouse |
| Preston-on-the-Patuxent |
| Price-Miller House |
| Priest Neal's Mass House and Mill Site |
| Primrose Hill |
| Princess Anne Historic District |
| Pritchard, Dr. William B., House |
| Proctor House |
| Prospect Hall |
| Prospect Hill |
Data from United States Park Service and Wikidata
