Iowa - Criteria: A (event)
| Octagon Round Barn, Indian Creek Township |
| Odd Fellows Block |
| Odenweller, F. F.-James P. and Nettie Morey House |
| Ogburn, William, House |
| Old Allamakee County Courthouse |
| Old Capitol |
| Old Chapel Hall |
| Old City Hall |
| Old East Paint Creek Lutheran Church |
| Old Fort Madison Site |
| Old Fourth Ward Southeast Historic District |
| Old Harrison County Courthouse |
| Old Lenox College |
| Old Settlers' Association of Johnson County Cabins |
| Old Town Historic District |
| Oleson Park Music Pavilion |
| Onawa IOOF Opera House |
| Orpheum Theatre and Site |
| Osage Commercial Historic District |
| Osceola Commercial Historic District |
| Osceola Masonic Block |
| Oskaloosa City Park and Band Stand |
| Oskaloosa Monthly Meeting of Friends Parsonage |
| Ottumwa Young Women's Christian Association |
| Overland Waterloo Company Building |
| Owl's Head Historic District |
| Oxford Commercial Historic District |
| Painter-Bernatz Mill |
| Palmer, B. J., House |
| Paramount Theater Building |
| Parker's Woods Park Historic District |
| Park/Glen Avenues Historic District |
| Park Hotel |
| Park Motel |
| Park-to-Park Residential Historic District |
| Penn College Historic District |
| Pentacrest |
| Perry and Brainard Block |
| Perry Carnegie Library Building |
| Petersen's, J. H. C., Sons Store |
| Peterson Manufacturing Building |
| Pierce School No. 13 |
| Pierson-Betts House |
| Pikes Point State Park Shelter and Steps |
| Pillsbury Point State Park |
| Pilotburg Church |
| Pilot Grove |
| Pine Mills German Methodist Episcopal Church |
| Pioneer Implement Company |
| Pioneer Oil Company Filling Station |
| Plagmann Round Barn |
| Pleasant Grove Community Church and Cemetery |
| Pleasant Hill School |
| Pleasant Lawn School Historic District |
| Plymouth County Courthouse |
| Podhajsky-Jansa Farmstead District |
| Poldberg, Chris, Farmstead |
| Polk County Homestead and Trust Company Addition Historic District |
| Polled Hereford Breed Origin Site |
| Polygonal Barn, Lincoln Township |
| Polygonal Barn, New Oregon Township |
| Polygonal Barn, Van Buren Township |
| Poplar Rural District |
| Pottawattamie County Jail |
| Prairie Center Methodist Episcopal Church and Pleasand Hill Cemetery |
| President's Cottage |
| Primrose Mill |
| Prospect Park Second Plat Historic District |
| Public Square Historic District |
| Pucker Street Historic District |
| Putnam-Parker Block |
| Quasdorf Blacksmith and Wagon Shop |
| Queen, Hogan and Martha A. Runkle, House |
| Ramsey Building |
| Rath Packing Company Administration Building |
| Reburn, Thomas, Polygonal Barn |
| Red Ball Garage |
| Redmond Park-Grande Avenue Historic District |
| Red Oak Downtown Historic District |
| Red Oak Firehouse and City Jail |
| Red Oak Grove Presbyterian Church and Cemetery |
| Reeve Electric Association Plant |
| Register and Tribune Building |
| Renwick Generating Plant |
| Reverend Little's Young Ladies Seminary |
| Reynolds, Anson O., House |
| Rhodes Mill |
| Riverview Park Plat Historic District |
| Roberts Octagon Barn |
| Rock Crest-Rock Glen Historic District |
| Rockford Mill |
| Rogers, George W. Company Shot Tower |
| Roosevelt Elementary School |
| Roosevelt School |
| Rose Hill Historic District |
| Roshek Brothers Department Store |
| Ross Grain Elevator |
| Round Barn, Bruce Township Section 3 |
| Round Barn, Buckingham Township |
| Round Barn, Cooper Township |
Data from United States Park Service and Wikidata
