- Omaha Structural Steel Works1
- One Minute Manufacturing Co.1
- Oneida Steel Pulley Co.2
- Ontario Biscuit Company1
- Orr, Painter & Co.1
- Otto Jaeger & Son, Inc.1
- Ottumwa Iron Works1
- Overland Hotel1
- Overman & Schrader Cordage Co.1
- Oxweld Acetylene Company1
- P. & F. Corbin1
- P. A. Canfield1
- P. A. Deyo & Son1
- P. J. Sorg Company1
- P.K. Dederick & Co.1
- Packard & Co.1
- Packard Hardware Co.2
- Page Belting Company1
- Page Bros. & Co.1
- Paine's Furniture Co.2
- Palace Hotel1
- Palmer House1
- Palmer, Bachelders & Co.1
- Paris Factory1
- Park Avenue Hotel3
- Park Steel Company1
- Park, Brother & Co., Limited.1
- Parker, Holmes & Co.5
- Parkersburg Mill Co.1
- Parkinson Coke & Coal Co.1
- Parlin & Orendorff Co.2
- Parry Mfg. Co.1
- Patterson Brothers1
- Payne's Bolt Works2
- Pecora Paint Company1
- Peerless Manufacturing Co.2
- Peirce College of Business1
- Penberthy Injector Co.1
- Peninsular Stove Company1
- Pennsylvania Bolt & Nut Co.4
- People's Storage Warehouse1
- Peoples National Bank1
- Perkins Wind Mill & Ax Co.1
- Perry & Brother1
- Peru Electric Mfg. Co.1
- Peter Hobart, Jr.2
- Peter Welty & Co.2
- Peters Cartridge Company1
- Petre, Schmidt & Bergmann1
- Peycke Bros. Co.4
- Phelps, Dodge & Co.1
- Phenix Iron Foundry1
- Philadelphia Gas Works1
- Philip Gordon1
- Phillip Carey M'f'g Co.1
- Phillip Carey Mfg. Co.1
- Phoenix1
- Piedmont Manufacturing Company2
- Pierce, Simmons & Co. Bankers1
- Pingree Company1