| Author | Titles | | 1.
| Ortega y Gasset, José, 1883-1955 | 27 | | 2.
| Ortega y Medina, Juan A. | 1 | | 3.
| Ortega-Morejón, José María de | 1 | | 4.
| Ortego, T. | 1 | | 5.
| Ortel, Philippe | 2 | | 6.
| Ortelius, Abraham, 1527-1598 | 5 | | 7.
| Ortero Urtaza, Fernando | 1 | | 8.
| Orthner, Franz | 2 | | 9.
| Orthwine, Rudolf | 1 | | 10.
| Orti, Alfredo Dagli | 1 |
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