| Open Invention Network | business, enterprise | Durham, New York City | November 10, 2005 |
| OpenAI | enterprise | San Francisco | December 11, 2015 |
| OpenTable | business | San Francisco | 1998 |
| Oracle Corporation | business, public company | Austin | June 16, 1977 |
| Oral Roberts University | church college, private not-for-profit educational institution, university | — | 1963 |
| Orange Julius | restaurant chain | Los Angeles, Edina | 1926 |
| Orbcomm | business, public company | Fort Lee | 1993 |
| Orbital ATK | launch service provider, former entity, defense contractor, aerospace manufacturer | Dulles | February 9, 2015 |
| Organon & Co. | organization, business, public company | Oss | 1923 |
| OTCBB | stock exchange, business | Washington, D.C. | — |
| Otis Worldwide Corporation | business, public company | Farmington | 1853 |
| Outerbanks Entertainment | television production company, film production company | Los Angeles | June 29, 1995 |
| Ovation Guitar Company | business | New Hartford | 1966 |
| Overbrook Entertainment | business | Beverly Hills, Los Angeles | 1998 |
| Ovintiv | public company | Denver | 2020 |
| O’Reilly Media | book publisher, publishing house | Sebastopol | 1978 |
| P. F. Chang's China Bistro | restaurant chain | — | 1993 |
| P.A. Semi | business | Santa Clara | 2003 |
| PACCAR | commercial vehicle manufacturer, enterprise, public company | Bellevue | 1905 |
| Pace University | private not-for-profit educational institution, university | New York City | 1906 |
| Packard Bell | technology company, brand, business, enterprise | Chatsworth | 1926 |
| Padlet | business, enterprise | San Francisco | December 1, 2012 |
| Palace Hotel | hotel | — | 1875 |
| Palantir Technologies | defense contractor, business, enterprise, public company | Miami | 2003 |
| Palo Alto Research Center | business | Palo Alto | 1970 |
| Pan Am | airline, business | New York City | March 14, 1927 |
| Pan American Health Organization | open-access publisher, international organization | Washington, D.C. | December 2, 1902 |
| Panda Express | fast food restaurant chain | Rosemead | 1973 |
| Panera Bread | restaurant chain, public company | Sunset Hills | 1987 |
| Panic | video game publisher, video game developer, business | Portland, Oregon | — |
| Panoz Auto Development | business, enterprise, automobile manufacturer | Hoschton | 1989 |
| Pantone | business, enterprise | Carlstadt | 1962 |
| Papa John's | fast food restaurant chain, pizzeria chain, pizza delivery, public company | Louisville | October 2, 1984 |
| Paramount Animation | business | Los Angeles | 2011 |
| Paramount Television Studios | television production company | Los Angeles | August 8, 2025 |
| Paramount+ | service on Internet, television channel, video streaming service, enterprise | New York City | October 28, 2014 |
| Paris Las Vegas | casino, hotel | — | September 1, 1999 |
| Parker Brothers | video game publisher, video game developer, board game publishing company, brand, business | Beverly, Salem | 1883 |
| Parker Pen Company | business | Newhaven, London | 1888 |
| Parsons School of Design | college, design school | — | 1936 |
| Payoneer | business | New York City | 2005 |
| Peabody Energy | business, enterprise, public company | St. Louis | 1883 |
| Peavey Electronics | business, enterprise | Meridian | 1965 |
| Peerless | business, defunct organization, automobile manufacturer | Cleveland | 1900 |
| Pegasystems | business, public company | Cambridge | 1983 |
| Penguin Random House | publisher, book publisher, organization, business, company | New York City | July 1, 2013 |
| Penn Central Transportation Company | railway company | Philadelphia | 1968 |
| Pennsylvania Hospital | hospital, teaching hospital | — | May 11, 1751 |