Illinois - Criteria: C (architecture/engineering)
| Old Danish Church |
| Old Iroquois County Courthouse |
| Old Main |
| Old Main, Augustana College |
| Old Market House |
| Old McHenry County Courthouse |
| Old Second National Bank |
| Old Stone Arch Bridge |
| Old Stone Arch, National Road |
| Old Stone Hotel |
| Old Town Triangle Historic District |
| Oliver Building |
| Olney Carnegie Library |
| Olympia Fields Country Club |
| One LaSalle Street Building |
| One Thirty North Eighth Building |
| Orchestra Hall |
| Oregon Commercial Historic District |
| Oregon Public Library |
| Orendorff, Ulysses G., House |
| Original Springs Hotel and Bathhouse |
| Orth House |
| Ottawa Commercial Historic District |
| Ottawa East Side Historic District |
| Otte, Dennis, Round Barn |
| Oughton, John R., House |
| Ouilmette North Historic District |
| Overton, Anthony, Elementary School |
| Pacesetter Gardens Historic District |
| Paddock, Henry I., House |
| Page Brothers Building |
| Page, Henry H., House |
| Palestine Commercial Historic District |
| Palmer, Col. Gustavus A., House |
| Palmer, Hiram, House |
| Palmer Park |
| Palmolive Building |
| Paramount Theatre |
| Paris Carnegie Public Library |
| Paris Elks Lodge No. 812 Building |
| Passionist Fathers Monastery |
| Patten, Charles H., House |
| Pattington Apartments |
| Patton Block Building |
| Paxton Carnegie Public Library |
| Paxton Water Tower and Pump House |
| Peabody, Elizabeth, School |
| Peabody, Francis Stuyvesant, Estate |
| Peace and Harvest |
| Peacock Brewery |
| Pekin Federal Building |
| Pelton, Ora, House |
| Pennsylvania Avenue Historic District |
| Pentecost, John L., House |
| Peoples Gas Building |
| Peoples National Bank Building-Fries Building |
| People's State Bank |
| Peoria City Hall |
| Peoria Cordage Company |
| Peoria Lock and Dam Historic District |
| Peoria Mineral Springs |
| Peoria State Hospital |
| Peoria Waterworks |
| Peotone Mill |
| Pere Marquette Hotel |
| Pere Marquette State Park Lodge and Cabins |
| Perkins, Dwight, House |
| Petersburg Historic District |
| Pettingill-Morron House |
| Pettit Memorial Chapel |
| Phi Delta Theta Fraternity House |
| Phi Mu Sorority House |
| Phoenix Opera House Block |
| Pick, George, House |
| Pickwick Theater Building |
| Pike-Sheldon House |
| Pilsen Historic District |
| Pine Craig |
| Pinehill |
| Piney Creek Site |
| Piney Creek South Site |
| Piney Creek West Site |
| Pioneer Gothic Church |
| Pioneers, The |
| Pittsfield East School |
| Pittsfield Historic District |
| Pleasant Grove School |
| Pleasant Home |
| Poinsetta Apartments |
| Polo Independent Order of Odd Fellows Lodge No. 197 |
| Pontiac Building |
| Portage Park |
| Portage Park Bungalow Historic District |
| Post House |
| Potter and Barker Grain Elevator |
| Prairie Avenue District |
| Prairie Dell Meetinghouse |
| Price-Prather House |
| Price/Wheeler House |
| Princeton Chapter House |
Data from United States Park Service and Wikidata
