Encyclopedias, Almanacs, Transcripts, and Maps
- Okara
- Okavango
- Okeechobee Waterway
- Okefenokee Swamp
- Okeghem, Jean d'
- Okhrida
- Oklahoma State University
- Olaf Guthfrithson
- Olaf V
- Olah, George Andrew
- Old Castile
- old English sheepdog
- Old Faithful
- Old Lyme
- Old Man of the Mountain (in Islamic history)
- Old Man of the Mountain (rock formation, United States)
- Old Orchard Beach
- Old Pretender
- Old Stone Age
- old-age pension