Illinois - Area of significance: Architecture
| Octagon House |
| Ogle County Courthouse |
| O'Grady, Mr. J. William de Coursey, House |
| Old Cahokia Courthouse |
| Old Chicago Historical Society Building |
| Old Colony Buildings |
| Old Customhouse |
| Old Danish Church |
| Old Iroquois County Courthouse |
| Old Main |
| Old Main, Almira College |
| Old Main, Augustana College |
| Old Market House |
| Old McHenry County Courthouse |
| Old Second National Bank |
| 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 |
| Ouilmette North Historic District |
| Our Lady of Lourdes Parish |
| Our Lady of the Holy Rosary Church/Holy Rosary Church |
| Overton, Anthony, Elementary School |
| Pacesetter Gardens Historic District |
| Paddock, Colonel George H., House |
| 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 |
| Paradise Missionary Baptist Church |
| Paramount Theatre |
| Paris Carnegie Public Library |
| Paris Elks Lodge No. 812 Building |
| Paris High School and Gymnasium |
| Parkway Garden Homes |
| 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 |
| 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 Mineral Springs |
| Peoria State Hospital |
| Peoria Waterworks |
| Peotone Mill |
| Pere Marquette Hotel |
| Pere Marquette State Park Lodge and Cabins |
| Perkins, Dwight, House |
| Perry, Joseph Ferris, House |
| Petersburg Historic District |
| Pettingill-Morron House |
| Pettit Memorial Chapel |
| Phi Delta Theta Fraternity House |
| Phillips, Alfred, House |
| Phi Mu Sorority House |
| Phoenix Opera House Block |
| Pick, George, House |
| Pickwick Theater Building |
| Pike-Sheldon House |
| Pilsen Historic District |
| Pine Craig |
| Pinehill |
| Pioneer Gothic Church |
| Pioneer Trust and Savings Bank |
| 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 |
Data from United States Park Service and Wikidata
