Encyclopedias, Almanacs, Transcripts, and Maps
- Nyssa
- Nystad
- Næstved
- O Nuallaian, Brian
- O'Brien, Conor Cruise
- O'Brien, Fitz-James
- O'Brien, Lucius Richard
- O'Brien, William Shoney
- O'Donnell, Leopoldo
- O'Donojú, Juan
- O'Grady, Standish Hayes
- O'Keefe, John
- O'Mahony, John
- O'Malley, Frank Ward
- O'Meara, Barry Edward
- O'Neill, Margaret
- O'Sullivan Dam
- O'Sullivan, Timothy H.
- O-shima
- Oahe Dam