- Oregon State Highway Commission (Salem, Or) (2)
- Organización Nacional de Latina Lesbianas y Latino Gais (LLEGO), National Latino/a Lesbian & Gay Organization; The National Task Force on AIDS Prevention (4)
- Ortus sanitatis. Moguntiae: J. Mayenbach (Mainz) (109)
- Orville Brewer (Chicago), 1904 (1)
- Osgood (Boston), 1864 (1)
- Ostdiek Co. (1)
- Otis Litho. Co. (1)
- Otis Lithograph Co. (6)
- Ottawa : Issued by the Director of Public Information for the Department of Munitions and Supply. (1)
- Ottawa : Wartime Information Board (2)
- Otto Dinse (1)
- Otto Hinz (1)
- Otto Holzhauser (1)
- Otto Wefers (1)
- Over de voortteeling en wonderbaerlyke veranderingen der Surinaamsche insecten. Amsterdam : by Jean Frederic Bernard (Amsterdam) (2)
- Ovitt, S. W. (New Haven, Connecticut) (65)
- Owens-Corning Fiberglas Corporation (1)
- Oxford University Press (London) (1)
- Oxford University Press (London, New York), 1953 (1)
- Oxford University Press (New York) (1)