| Authors | Bibliographic Citations |
| O | O (1874) |
| O'Dubhgaill, Proinnsias | O'Dubhgaill (1937) |
| Oliver, G. | Oliver (1837) |
| Olivier, Edith [ed.] | Olivier & Edwards (1930) |
| "Omurethi" | Omurethi (1908) |
| Ordish, T. Fairman | Ordish (1891) - Ordish (1893) - Ordish (1899) - Ordish (1901) - Ordish Coll. |
| Ordnance Survey | Ordnance Survey (n.d.) |
| Ormerod, G. | Ormerod MSS |
| Ormerod, George | Ormerod (1882) |
| Ornsby, George [ed.] | Ornsby (1878) |
| O'Toole, Edward | O'Toole (1930) |
| Oxfordshire Federation of Women's Institutes | Oxfordshire Fed.W.I.s (1955) |