| O'Reilly Auto Parts | business, public company | Springfield | 1957 |
| Oakley, Inc. | business, subsidiary company, enterprise | Foothill Ranch | 1975 |
| Oberlin College | liberal arts college, college, private not-for-profit educational institution, school, private university | Oberlin | 1833 |
| Occidental Petroleum | business, public company | Houston | 1920 |
| Ocean Casino Resort | hotel | — | 2018 |
| Office Depot | business, retail chain, enterprise, public company | Boca Raton | 1986 |
| Ohio Valley Wrestling | business, enterprise, professional wrestling promotion | Louisville | 1997 |
| Oklahoma State University | public educational institution of the United States, land-grant university, public research university, sun grant institution | — | 1890 |
| Old Navy | business | San Francisco | 1994 |
| Olmsted Brothers | architectural firm, landscape architectural firm | Brookline | 1898 |
| Omega Aerial Refueling Services | airline | — | 2001 |
| Omni Air International | airline | Tulsa International Airport | March 15, 1993 |
| Oni Press | comics publishing company, book publisher | Portland | 1997 |
| Onsemi | business, enterprise, public company | Scottsdale | June 18, 1992 |
| Open Library | digital library | — | July 16, 2007 |
| 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 |
| Oral Roberts University | church college, private not-for-profit educational institution, university | — | 1963 |
| Orbcomm | business, public company | Fort Lee | 1993 |
| Oregon Health and Science University | public educational institution of the United States, public university | Portland | 1974 |
| Oregon State University | research university, sea grant institution, space grant institution, public educational institution of the United States, land-grant university, public research university, sun grant institution, public university | — | 1856 |
| Organon & Co. | organization, business, public company | Oss | 1923 |
| Origin Systems | video game publisher, video game developer | Austin | 1983 |
| Original Film | film production company | Los Angeles | 1997 |
| Orion Pictures | film production company, business | Los Angeles | January 1978 |
| Osborne Computer Corporation | business | Hayward | September 27, 1979 |
| Oscar Mayer | business | Chicago | 1883 |
| Oshkosh Corporation | business, enterprise, public company | Oshkosh | 1917 |
| Overbrook Entertainment | business | Beverly Hills, Los Angeles | 1998 |