Encyclopedias, Almanacs, Transcripts, and Maps
- Oblate Sisters of the Most Holy Redeemer
- Oblates of Mary Immaculate
- Oblates of St. Charles
- Oblates of St. Francis de Sales
- Oblates of St. Joseph
- Obligation, Moral
- Obligations, Law of
- Obrecht, Jakob
- Obrecht, M. Edmond
- Obregonians (Poor Infirmarians)
- Ocaña
- Occasion
- Occult Orders
- Occult Orders
- Oceania, The Catholic Church in
- Ochberg, Isaac
- Ochino, Bernardino
- Ochrida
- Ochs
- Ochs, Philip David