| O'Reilly Auto Parts | business, public company | Springfield | 1957 |
| O.F. Mossberg & Sons | business, enterprise | North Haven | 1919 |
| Oakland Motor Car Company | automobile manufacturer | Pontiac | 1907 |
| Oberheim Electronics | business | Oakland | 1969 |
| Obsidian Entertainment | video game publisher, video game developer, business | Irvine | June 12, 2003 |
| Occidental Petroleum | business, public company | Houston | 1920 |
| Ocean Casino Resort | hotel | — | 2018 |
| Ocean Spray | agricultural cooperative, business | Lakeville | 1930 |
| Office Depot | business, retail chain, enterprise, public company | Boca Raton | 1986 |
| 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 |
| Oikos University | university | Oakland | 2004 |
| Old Navy | business | San Francisco | 1994 |
| Oldsmobile | car brand, division, automobile manufacturer | Lansing | 1897 |
| Olin Corporation | business, enterprise, public company | Clayton | 1892 |
| 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 |