Iowa - Criteria: C (architecture/engineering)
| 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 |
| Ossian Opera House |
| Otter Creek Bridge |
| Ottumwa Cemetery Historic District |
| Ottumwa Public Library |
| Ottumwa Young Women's Christian Association |
| Our Mother of Sorrows Grotto Historic District |
| Outing Club |
| Oversen, Julius and Anine House |
| Owl's Head Historic District |
| Oxford Commercial Historic District |
| Page County Courthouse |
| Pahl, Henry, 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 |
| Peak, George B., House |
| Pearson, Franklin, 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 |
| Pillsbury Point State Park |
| Pine Mill Bridge |
| Pioneer Oil Company Filling Station |
| Plagmann Round Barn |
| Plambeck, Joachim, House |
| Plane, Robert R., and Julia L., House |
| Pleasant Grove Community Church and Cemetery |
| Pleasant Grove Presbyterian Church |
| Plum Grove |
| Plymouth County Courthouse |
| Plymouth Place |
| Pocahontas County Courthouse |
| Podhajsky-Jansa Farmstead District |
| Pohlmann, Elizabeth, House |
| Pohlmann, Henry, House |
| Poldberg, Chris, Farmstead |
| Polk County Courthouse |
| Polk County Homestead and Trust Company Addition Historic District |
| Polygonal Barn, Lincoln Township |
| Polygonal Barn, New Oregon Township |
| Polygonal Barn, Van Buren Township |
| Poplar Rural District |
| Porter Hall |
| Porter-Rhynsburger House |
| Pottawattamie County Jail |
| Pottawattamie County Sub Courthouse |
| Potter's, E. G., Jasper Flour Mill |
| Potter-Williams House |
| Poweshiek County Courthouse |
| Pratt, A. W., House |
| President's Cottage |
| Price, Rialto, House |
| Prien Building |
| Priester Building, The |
| Primrose Mill |
| Princess-Sweet Shop |
| Prospect Park Historic District |
| Prospect Park Second Plat Historic District |
| Public Library of Des Moines |
| Public Square Historic District |
Data from United States Park Service and Wikidata
