| O'Reilly Auto Parts | Springfield | 1957 |
| Oakley, Inc. | Foothill Ranch | 1975 |
| Occidental Petroleum | Houston | 1920 |
| Office Depot | Boca Raton | 1986 |
| Ohio Valley Wrestling | Louisville | 1997 |
| Old Navy | San Francisco | 1994 |
| Onsemi | Scottsdale | June 18, 1992 |
| OpenDNS | San Francisco | July 2006 |
| OQO | San Francisco | 2000 |
| Orbcomm | Fort Lee | 1993 |
| Organon & Co. | Oss | 1923 |
| Orion Pictures | Los Angeles | January 1978 |
| Osborne Computer Corporation | Hayward | September 27, 1979 |
| Oscar Mayer | Chicago | 1883 |
| Oshkosh Corporation | Oshkosh | 1917 |
| Overbrook Entertainment | Beverly Hills, Los Angeles | 1998 |
| P.A. Semi | Santa Clara | 2003 |
| Pacific Data Images | Redwood City | 1980 |
| Packard Bell | Chatsworth | 1926 |
| Padlet | San Francisco | December 1, 2012 |
| Palantir Technologies | Miami | 2003 |
| Palm, Inc. | Sunnyvale | 1992 |
| Palo Alto Research Center | Palo Alto | 1970 |
| Pan Am | New York City | March 14, 1927 |
| Panic | Portland, Oregon | — |
| Pantene | Switzerland | 1945 |
| Pantone | Carlstadt | 1962 |
| Parallels | Renton | 1999 |
| Paramount Global Content Distribution | Santa Monica, New York City | 1962 |
| Parker Brothers | Beverly, Salem | 1883 |
| Peabody Energy | St. Louis | 1883 |
| Peavey Electronics | Meridian | 1965 |
| Pegasystems | Cambridge | 1983 |
| Pennzoil | Houston | 1963 |
| Penske Media Corporation | Los Angeles | 2004 |
| Pentaho | Orlando | 2004 |
| People Incorporated | New York City | July 28, 1986 |
| Perdue Farms | Salisbury | 1920 |
| Perforce Zend | Minneapolis | 1999 |
| Perplexity AI | San Francisco | December 7, 2022 |
| PetSmart | Phoenix | 1986 |
| Phillips 66 | Westchase | May 1, 2012 |
| PhoenixEV | Ontario, California | 2002 |
| Pilgrim's Corp | Greeley | 1946 |
| PIMCO | Newport Beach | 1971 |
| Pinkerton Government Services | Ann Arbor | February 1855 |
| Pinnacle Systems | Mountain View | 1986 |
| Pioneer Hi-Bred | Johnston | 1926 |