| Olive Garden | restaurant chain | Orlando | December 13, 1982 |
| One America News Network | news website, television channel, organization, subscription television, cable news in the United States | San Diego | July 4, 2013 |
| ONErpm | record label, music distributor, business | Brooklyn | 2010 |
| Oneworld | airline alliance, business | Fort Worth | 1999 |
| Open Invention Network | business, enterprise | Durham, New York City | November 10, 2005 |
| Open Road Flims | film production company, film distributor | Los Angeles | 2011 |
| OpenDNS | service on Internet, public recursive name server, business | San Francisco | July 2006 |
| OQO | business, enterprise | San Francisco | 2000 |
| Orbital ATK | launch service provider, former entity, defense contractor, aerospace manufacturer | Dulles | February 9, 2015 |
| Orbital Sciences Corporation | launch service provider, enterprise, public company, aerospace manufacturer | Dulles | 1982 |
| Oregon Institute of Technology | university of technology, public educational institution of the United States | — | 1947 |
| Orion Pictures | film production company, business | Los Angeles | January 1978 |
| Oscar Mayer | business | Chicago | 1883 |
| Oshkosh Corporation | business, enterprise, public company | Oshkosh | 1917 |
| Otis Worldwide Corporation | business, public company | Farmington | 1853 |
| Ovation Guitar Company | business | New Hartford | 1966 |
| Overbrook Entertainment | business | Beverly Hills, Los Angeles | 1998 |
| P. F. Chang's China Bistro | restaurant chain | — | 1993 |
| Pacific Data Images | business | Redwood City | 1980 |
| Pacific Gas and Electric Company | public company | Pacific Gas & Electric Building, San Francisco | 1905 |
| Packard Bell | technology company, brand, business, enterprise | Chatsworth | 1926 |
| Palace Theatre | Broadway theatre, theatre building, movie theater, music venue | — | March 24, 1913 |
| Palm, Inc. | business, enterprise | Sunnyvale | 1992 |
| Palms Casino Resort | skyscraper, casino, resort | — | 2001 |
| 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 |
| Panera Bread | restaurant chain, public company | Sunset Hills | 1987 |
| Panic | video game publisher, video game developer, business | Portland, Oregon | — |
| Paper Mate | business | Oak Brook | 1940 |
| Parametric Technology Corporation | business, enterprise, public company | Boston | May 1985 |
| Paramount Home Entertainment | film distributor, subsidiary company | Hollywood | 1975 |
| Paramount+ | service on Internet, television channel, video streaming service, enterprise | New York City | October 28, 2014 |
| Paris Las Vegas | casino, hotel | — | September 1, 1999 |
| Park MGM | skyscraper, casino, hotel, resort | — | 1996 |
| Parker Brothers | video game publisher, video game developer, board game publishing company, brand, business | Beverly, Salem | 1883 |
| Parsons School of Design | college, design school | — | 1936 |
| Participant | production company, business | Los Angeles | January 2004 |
| Patagonia, Inc. | privately held company, business, retail chain, enterprise | Ventura | 1973 |
| Peabody Institute | conservatory, school | Baltimore | 1857 |
| Peavey Electronics | business, enterprise | Meridian | 1965 |
| Pei Wei Asian Diner | restaurant | Scottsdale | 2000 |
| Penguin Random House | publisher, book publisher, organization, business, company | New York City | July 1, 2013 |
| Pennsylvania Railroad | railway company, business | Philadelphia | April 13, 1846 |
| Pennsylvania State University | public research university | Penn State University Park | February 22, 1855 |
| Penske Media Corporation | media company, business | Los Angeles | 2004 |
| Perdue Farms | business | Salisbury | 1920 |
| PerkinElmer | organization, business, company | Waltham | 1937 |
| Perplexity AI | artificial intelligence website, organization, business | San Francisco | December 7, 2022 |