Encyclopedias, Almanacs, Transcripts, and Maps
- Olaf I Tryggvason, King of Norway
- Olatunji, Michael Babatunde
- Old Believer Committee
- Old Man and Old Woman
- Old Style
- Oliphant, Mark
- Olivares, Gaspar De Guzmán Y Pimentel, Count of (1587–1645)
- Olive Branch Petition
- Olive Branch Petition
- Oliver P. Smith
- Olorun
- Olympia Snowe
- Olympians
- Omar al-Mukhtar
- Omar ibn al-Khattab
- Omar N. Bradley
- Omnibus Crime Control and Safe Streets Act of 1968
- On Command
- On the Lines
- Ong Teng Cheong