Encyclopedias, Almanacs, Transcripts, and Maps
- 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'
- Orléans, Île d'
- Orlov, Aleksey Grigoryevich, Count
- Orlov, Grigori Grigoryevich, Count
- Orly
- Ormazd
- Ormerod, Eleanor Anne
- Ormond Beach
- Ormonde, James Butler, 5th earl of