- Opúsculos por Alexandre Herculano - Tomo 05 (Portuguese) by Herculano, Alexandre
- Opúsculos por Alexandre Herculano - Tomo 07 (Portuguese) by Herculano, Alexandre
- Opúsculos por Alexandre Herculano - Tomo 09 (Portuguese) by Herculano, Alexandre
- Op de Faröer - De Aarde en haar Volken, 1907 (Dutch) by Seé, Anna
- Op de jacht in Mozambique - De Aarde en haar Volken, 1909 (Dutch) by Vasse, Guillaume
- Op de olifantenjacht in Oeganda - De Aarde en haar Volken, 1910 (Dutch) by Langsdorff, Baron de
- The Open Air (English) by Jefferies, Richard
- The Open Door, and the Portrait. - Stories of the Seen and the Unseen. (English) by Oliphant, Mrs. (Margaret)
- An Open-Eyed Conspiracy; an Idyl of Saratoga (English) by Howells, William Dean
- Opening a Chestnut Burr (English) by Roe, Edward Payson
- Openings in the Old Trail (English) by Harte, Bret
- An Open Letter on Translating (English) by Luther, Martin
- The Open Secret of Ireland (English) by Kettle, T. M. (Thomas Michael)
- Open Source Democracy (English) by Rushkoff, Douglas
- The Opera - A Sketch of the Development of Opera. With full Descriptions of all Works in the Modern Repertory. (English) by Fuller-Maitland, J. A.
- The Opera - A Sketch of the Development of Opera. With full Descriptions of all Works in the Modern Repertory. (English) by Streatfeild, R. A. (Richard Alexander)
- Opera Reel with Calls (English) by Bisbee, Jasper (Fiddle)
- Opera Stories from Wagner (English) by Akin, Florence
- Operation: Outer Space (English) by Leinster, Murray, [pseud.]
- Operation R.S.V.P. (English) by Piper, H. Beam
- Operation Terror (English) by Leinster, Murray, [pseud.]
- O Perfect Love (English) by Burleigh, H.T.
- The Opinions of a Philosopher (English) by Grant, Robert
- Opinions sociales (French) by France, Anatole
- O Pioneers! (English) by Cather, Willa Sibert
- The Opium Habit (English) by Day, Horace B.
- L'oppidum de Bibracte - Guide historique et archéologique au Mont Beuvray; d'après les documents archéologiques les plus récents (French) by Anonymous
- Oppikirja suomalaisen kirjallisuuden historiassa (Finnish) by Godenhjelm, Bernhard Frederick
- Op reis en thuis (Dutch) by Maurik, Justus van
- Op Samoa (Dutch) by Barfus, Eginhard von
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