Encyclopedias, Almanacs, Transcripts, and Maps
- Orthodox Church of Cyprus
- Orthodox Church of Georgia
- Orthodox Church of Greece
- Orthodox Church of Romania
- Orthodox Church of Russia
- Orthodox Church of Serbia
- Orthodox Judaism
- Orthodox Symbolic Books
- Ortiz de Zárate, Pedro, ven
- Ortlibarii
- Ortolana (Hortulana), Bl.
- Orton, William Aylott
- Orval, Abbey of
- Orvieto, Angiolo
- Orzeszkowa (Orzeszko), Eliza°
- Osbaldeston, Edward, Bl.
- Osbern of Gloucester
- Osborn, Max
- Oschinsky, Lawrence
- Oshaiah (Hoshaiah) Rabbah