Encyclopedias, Almanacs, Transcripts, and Maps
- O'Connell, William Henry
- O'Connor, Martin John
- O'Connor, Mary Flannery
- O'Connor, Michael
- O'Connor, Thomas Francis
- O'Cullenan, Gelasius (Glaisne)
- O'Daly, Daniel
- O'Daniel, Victor Francis
- O'Devany, Conor, Bl.
- O'Donnell, Edmund
- O'Fihely, Maurice
- O'Gorman, Thomas
- O'Hara, Edwin Vincent
- O'Hara, John Francis
- O'Hely, Patrick, Bl.
- O'Hurley, Dermot, Bl.
- O'Leary, Henry Joseph
- O'Neill, Sara Benedicta
- O'Reilly, Bernard
- O'Reilly, Edmund