Iowa - Area of significance: Architecture
| Oak Hill Cemetery Historic District |
| Oak Hill Historic District |
| Oaklands, The, Historic District |
| Oak Lane Historic District |
| O'Brien County Courthouse |
| Ockershausen, Henry, House |
| Odd Fellows Block |
| Odd Fellows Hall |
| Odd Fellows Hall |
| Ogburn, William, House |
| Old Allamakee County Courthouse |
| Old Capitol |
| Old East Paint Creek Lutheran Church |
| Old Fourth Ward Southeast Historic District |
| Old Harrison County Courthouse |
| Old Jail |
| Old Lenox College |
| Old Main |
| Old Main Historic District |
| Old Mill House |
| Old Post Office |
| Old Settlers' Association of Johnson County Cabins |
| Old Town Historic District |
| Old Worth County Courthouse |
| Oleson Park Music Pavilion |
| Onawa Public Library |
| Organ, Alexander, House |
| Orpheum Theatre and Site |
| Osage Commercial Historic District |
| Osborne, Edmund B., House |
| Osceola Commercial Historic District |
| Osceola County Courthouse |
| Osceola Masonic Block |
| Oskaloosa City Hall |
| Oskaloosa City Park and Band Stand |
| Oskaloosa City Square Commercial Historic District |
| Oskaloosa Fire Station |
| Oskaloosa Monthly Meeting of Friends Parsonage |
| Oskaloosa Post Office |
| Ossian Opera House |
| Ottumwa Cemetery Historic District |
| Ottumwa Public Library |
| Ottumwa Young Women's Christian Association |
| Our Mother of Sorrows Grotto Historic District |
| Our Savior’s Lutheran Church |
| Outing Club |
| Oversen, Julius and Anine House |
| Owl's Head Historic District |
| Oxford Commercial Historic District |
| Page County Courthouse |
| Pahl, Henry, House |
| Paine, Samuel and Mercy, House |
| Palmer, B. J., House |
| Palmyra Methodist Episcopal Church |
| Paradise Block Historic District |
| Paramount Theater Building |
| Parker House |
| Parker's Opera House |
| Park/Glen Avenues Historic District |
| Park Hotel |
| Park House Hotel |
| Park Inn Hotel |
| Park Motel |
| Park-to-Park Residential Historic District |
| Paul-Helen Building |
| Paulsen, Peter J., House |
| Paustian, Henry, House |
| Payne, O. E., House |
| Pearson, Franklin, House |
| Pease, Dr. Luther L. and Susette E. (Baker), House |
| Peirce, John, House |
| Pella Opera House |
| Penn College Historic District |
| Pentacrest |
| Peoples Nationals Bank |
| People's Savings Bank |
| Perham House |
| Perkins, Charles W. and Nellie, House |
| Perkins, George and Lola, House |
| Perrigo-Holmes House |
| Perry Carnegie Library Building |
| Perry, T. B., House |
| Peters' Barber Shop |
| Petersen, Max, House |
| Petersen's, J. H. C., Sons Store |
| Petersen, W. D., Memorial Music Pavillion |
| Peters, J. C., House |
| Picklum, Frank, House |
| Pierce School No. 13 |
| Pikes Point State Park Shelter and Steps |
| Pilot Knob State Park, Observation Tower (Area 2) |
| Pilot Knob State Park, Picnic Shelter (Area 3) |
| Pioneer Oil Company Filling Station |
| Plambeck, Joachim, House |
| Plane, Robert R., and Julia L., House |
| Pleasant Grove Community Church and Cemetery |
| Pleasant Grove Presbyterian Church |
| Pleasant View Stock Farm Historic District |
| Plum Grove |
| Plymouth County Courthouse |
Data from United States Park Service and Wikidata
