O.
- Occurring insanity, [260]
- O'Curry, Professor, [26]
- Ogilvie, Sir John, [308], [356]
- O'Hagan, Lord, [432], [434], [435], [437], [441]
- Oleum cephalicum, [93]
- Oliver Cromwell's porter, [71]
- Omagh Asylum, [421]
- Open-door system, [376-378]
- Orange, Dr. William, [273], [276], [494]
- Osborne, Dr., [407]
- Overall, Mr., [60], [507]