| Oberheim Electronics | business | Oakland | 1969 |
| Oberlin College | liberal arts college, college, private not-for-profit educational institution, school, private university | Oberlin | 1833 |
| Obsidian Entertainment | video game publisher, video game developer, business | Irvine | June 12, 2003 |
| Occidental Petroleum | business, public company | Houston | 1920 |
| OceanGate, Inc. | business | Everett | 2009 |
| Oddworld Inhabitants | video game developer | Berkeley | 1994 |
| Odyssey Marine Exploration | business, public company | — | 1994 |
| OfficeMax | business, retail chain | — | 1988 |
| Ogilvy | public relations agency, advertising agency | New York City | 1948 |
| Ohio University | public educational institution of the United States, public university | Athens | 1804 |
| Ohio Valley Wrestling | business, enterprise, professional wrestling promotion | Louisville | 1997 |
| Ohio Wesleyan University | liberal arts college, Colleges That Change Lives, private not-for-profit educational institution | Delaware | 1842 |
| Oklahoma State University | public educational institution of the United States, land-grant university, public research university, sun grant institution | — | 1890 |
| Old Dominion University | sea grant institution, public educational institution of the United States, university | — | 1930 |
| Olive Garden | restaurant chain | Orlando | December 13, 1982 |
| Omega Aerial Refueling Services | airline | — | 2001 |
| Omnicom Group | advertising agency, business, public company | New York City | 1986 |
| One America News Network | news website, television channel, organization, subscription television, cable news in the United States | San Diego | July 4, 2013 |
| One Race Films | enterprise | Studio City | 1995 |
| Onsemi | business, enterprise, public company | Scottsdale | June 18, 1992 |
| 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 |