Encyclopedias, Almanacs, Transcripts, and Maps
- Oregon State University
- Orekhovo-Zuyevo
- Orem
- Oreopithecus
- Oreor
- Orestia
- Orford, Robert Walpole, 1st earl of
- Organ Pipe Cactus National Monument
- organ transplant
- Organization for the Prohibition of Chemical Weapons
- Organization of Eastern Caribbean States
- Organization of the Islamic Conference
- organometallic chemistry
- Orhan
- Oribe, Manuel
- oribi
- Orientale
- Orkhan
- Orléanais
- Orléans, Henrietta Anne, duchesse d'