Encyclopedias, Almanacs, Transcripts, and Maps
- Oklahoma City, Archdiocese of
- Oklahoma, Catholic Church in
- Oko, Adolph S.
- Olaf I Tryggvessøn, King of Norway
- Olaf II, King of Norway, St.
- Olah, George A.
- Oláh, Miklós (Olahus)
- Olam Ha-Ba
- Olan, Levi Arthur
- Olavide y Jauregui, Pablo de
- Olbracht, Ivan
- Old Chapter
- Old Deluder Satan Act
- Old Roman Chant
- Oldegar, St.
- Oldenburg
- Oldmeadow, Ernest James
- Olei Ha-Gardom
- Olesko
- Olesnica