| O'Reilly Auto Parts | Springfield | 1957 |
| Oakley, Inc. | Foothill Ranch | 1975 |
| Obsidian Entertainment | Irvine | 12 de junio de 2003 |
| Ocean Spray | Lakeville | 1930 |
| OceanGate | Everett | 2009 |
| Odyssey Marine Exploration | Tampa | 1994 |
| Office Depot | Boca Raton | 1986 |
| OfficeMax | — | 1988 |
| ON Semiconductor | Scottsdale | 18 de junio de 1992 |
| OpenDNS | San Francisco | julio 2006 |
| OpenTable | San Francisco | 1998 |
| OQO | San Francisco | 2000 |
| Orbcomm | Fort Lee | 1993 |
| Orion Pictures | Los Ángeles | enero 1978 |
| Osborne Computer Corporation | Hayward | 27 de septiembre de 1979 |
| Oscar Mayer | Chicago | 1883 |
| Overbrook Entertainment | Beverly Hills, Los Ángeles | 1998 |
| Overstock.com | Midvale | 2023 |
| Owen illinois | Toledo, Perrysburg | 1903 |
| Packard Bell | Chatsworth | 1926 |
| PalmSource | Reston | — |
| Pan Am | Nueva York | 14 de marzo de 1927 |
| Panavision | Woodland Hills | 1953 |
| Panoz | Hoschton | 1989 |
| Pantene | Suiza | 1945 |
| Pantone | Carlstadt | 1962 |
| Paper Mate | Oak Brook | 1940 |
| Paramount Animation | Los Ángeles | 2011 |
| Parker Brothers | Beverly, Salem | 1883 |
| Parker Pen Company | Newhaven, Londres | 1888 |
| Parkwood Entertainment | Los Ángeles | 2008 |
| Payoneer | Nueva York | 2005 |
| Peabody Energy | San Luis | 1883 |
| Peerless | Cleveland | 1900 |
| Perdue | Salisbury | 1920 |
| PhoenixEV | Ontario | 2002 |
| Pierce-Arrow | Búfalo | 1870 |
| Pinkberry | Los Ángeles | 2005 |
| Pioneer Hi-Bred | Johnston | 1926 |
| Plantronics | Santa Cruz | 1961 |
| Playboy Enterprises | Los Ángeles | 1953 |
| Plug Power | Latham, Slingerlands | 1997 |
| PNC Financial Services | One PNC Plaza | 1982 |
| Polycom | San José | 1990 |
| Powell Peralta | — | 1978 |
| Powerset | San Francisco | 2005 |
| Pratt & Whitney Rocketdyne | Canoga Park | 2005 |
| Presupuesto Rent a Car | Chicago, Parsippany-Troy Hills | 1958 |