| Orient. Inst. Pubns | Oriental Institute Publications |
| Oxford Class. Dict. | M. Cary and others: The Oxford Classical Dictionary (Oxford, 1948); rev., N. G. L. Hammond and H. H. Scullard, eds (Oxford, 2/1970) |
| Oxford Hist. Eng. A. | The Oxford History of English Art |
| O’Donoghue | F. O’Donoghue and H. M. Hake: Catalogue of Engraved British Portraits Preserved in the Department of Prints and Drawings in the British Museum, 6 vols (London, 1908–25) |
| ÖBL | Österreichisches biographisches Lexikon (Graz, Cologne and Vienna, 1957–) |
| ÖKL | R. Schmidt, ed.: Österreichisches Künstlerlexikon von den Anfängen bis zur Gegenwart (Vienna, 1974–) |
