| North American Arms | business | Provo | 1972 |
| North American Aviation | former entity, aerospace manufacturer | Los Angeles | December 6, 1928 |
| North Carolina Agricultural and Technical State University | historically black college or university, public educational institution of the United States, land-grant university, public university | Greensboro | 1891 |
| North Carolina Central University | historically black college or university, public educational institution of the United States, university | Durham | 1909 |
| North Carolina State University | public educational institution of the United States, open-access publisher, land-grant university, public research university | Raleigh | 1887 |
| Northeast Airlines | airline | Boston | 1931 |
| Northeastern Illinois University | public educational institution of the United States, university | — | 1867 |
| Northeastern University | private not-for-profit educational institution, university | Boston | 1898 |
| Northern Air Cargo | airline | Anchorage | 1956 |
| Northern Arizona University | public educational institution of the United States, open-access publisher, public research university | — | 1899 |
| Northern Illinois University | public educational institution of the United States, university, normal school | DeKalb | 1895 |
| Northern Kentucky University | public educational institution of the United States, public university | — | 1968 |
| Northwest Airlines | airline | Eagan | 1926 |
| Northwestern Mutual | business | Milwaukee | 1857 |
| Norwich University | military university, private not-for-profit educational institution | — | 1819 |
| Nova Southeastern University | private not-for-profit educational institution, university, open-access publisher | Davie | 1964 |
| NovaKid | privately held company, software, company | San Francisco | 2017 |
| NovaLogic | video game developer | Agoura Hills | 1985 |
| NPR | radio network, public broadcaster, online newspaper without paywall, nonprofit organization, nonprofit corporation | Washington, D.C. | February 26, 1970 |
| NRG Energy | business, public company | Princeton, Houston | 1905 |
| NStigate Games | video game developer | Marin County | March 1998 |
| Nu Skin Enterprises | multi-level marketing company, public company | Provo | 1984 |
| NXIVM | cult, corporation, multi-level marketing company | Albany | July 20, 1998 |
| NZXT | business | Los Angeles | 2004 |
| 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 |