| Olivetti-Underwood | Hartford | CT |
| Oneco Manufacturing | New London | CT |
| Orford Soap | Manchester | CT |
| Otis Elevator | Farmington | CT |
| Owens Corning | East Hartford | CT |
| Palmer Brothers | Montville | CT |
| Park City Hospital | Bridgeport | CT |
| Park City Supply | Bridgeport | CT |
| Parkville Siding | Hartford | CT |
| Parts | East Hartford | CT |
| Patterson Steel & Forge | Stratford | CT |
| Pawcatuck Water Pollution | Pawcatuck | CT |
| Peabody Engineering | Stamford | CT |
| Peck Brothers & Company | New Haven | CT |
| Peck Stow & Wilcox | Southington | CT |
| Pedleton Fire Brick | Bloomfield | CT |
| Peerless Aluminum Fdy | Bridgeport | CT |
| Peoples Savings Bank, Main Street | Bridgeport | CT |
| Perramar Hotel | Old Saybrook | CT |
| Petroleum Heat & Power | Stamford | CT |
| Pfizer | Groton | CT |
| Pfizer | Wallingford | CT |
| Phillip Sporn | New Haven | CT |
| Phoenix Mutual Life Ins | Hartford | CT |
| Phoenix Mutual Life Ins | Wethersfield | CT |
| Pierce Generating Station | Wallingford | CT |
| Pitney Bowes | Danbury | CT |
| Plainfield Mills | Plainfield | CT |
| Plainville Elementary School | Plainville | CT |
| Plainvillle Casting | Plainville | CT |
| Plastic Film | Plainfield | CT |
| Plastic Molding | Sandy Hook | CT |
| Plastic Products | Norwalk | CT |
| Plasticrete | Hamden | CT |
| Plasticrete | New Haven | CT |
| Plasticrete Brick | Hamden | CT |
| Plume & Atwood Brass & Company | Thomaston | CT |
| Ponds Extract | Bridgeport | CT |
| Popular Market | Manchester | CT |
| Porter Box & Lumber | Bridgeport | CT |
| Post Publishing | Bridgeport | CT |
| Post Secondary Tech | Norwalk | CT |
| Potlatch | Windsor | CT |
| Powdrell & Alexander | Danielson | CT |
| Power Plant | Wallingford | CT |
| Pratt Bros & Company | Deep River | CT |
| Pratt, Read & Company | Essex | CT |
| Pratt & Whitney Aircraft | North Haven | CT |
| Pratt & Whitney Aircraft | East Hartford | CT |
| Pratt & Whitney Aircraft | Hartford | CT |
