- Oakdale Affair, The, by Edgar Rice Burroughs
- Oak Openings, by James Fenimore Cooper
- Oberon, by Christoph Martin Wieland
- Obiter Dicta, by Augustine Birrell
- Observations by Mr. Dooley, by Finley Peter Dunne
- Observations of a Retired Veteran, by Henry C. Tinsley
- Oceana, by James Harrington
- O'Conors of Castle Conor, by Anthony Trollope
- Octopus, The, by Frank Norris
- Odd Women, The, by George Gissing
- Oden, by Gotthold Ephraim Lessing
- Odes and Carmen Saeculare of Horace, by Horace
- Odes and Epodes, by Horace
- Odes and Epodes, by Horace
- Odyssey, The, by Homer
- Oedipus Trilogy, by Sophocles
- Of Captain Mission, by Daniel Defoe
- Off-Hand Sketches, by T. S. Arthur
- Official report of the exploration of the Queen Charlotte Islands for the government of British Columbia, by Newton H. Chittenden
- Off on a Comet, by Jules Verne
- Of Human Bondage, by Somerset Maugham
- O'Flaherty V.C., by George Bernard Shaw
- Of Literature (Entire), by W. D. Howells
- Of The Nature of Things, by Lucretius [Titus Lucretius Carus]
- Oh, Money! Money!, by Eleanor Hodgman Porter
- Okewood of the Secret Service, by Valentine Williams
- Olaf van Geldern, by Pencho Slaveykov
- Old Age and Death, by Jacques Casanova
- Old Bachelor, The, by William Congreve
- Old Calabria, by Norman Douglas
- Old Caravan Days, by Mary Hartwell Catherwood
- Old Christmas, by Washington Irving
- Old Curiosity Shop, The, by Charles Dickens
- Old English Baron, The, by Clara Reeve
- Old English Libraries, The Making, Collection, and Use of Books During the Middle Ages, by Ernest A. Savage
- Old French Romances, by William Morris
- Old Friends, by Andrew Lang
- Old Fritz and the New Era, by Muhlbach
- Old Granny Fox, by Thornton W. Burgess
- Old Gray Homestead, The, by Frances Parkinson Keyes
- Old Indian Days, by Charles Eastman
- Old Indian Legends, by Zitkala-Sa
- Old John Brown, by Walter Hawkins
- Old Love Stories Retold, by Richard Le Gallienne
- Old Merchant Marine, The, by Ralph D. Paine
- Old Mission Stories of California, by Charles Franklin Carter
- Old Mortality, by Walter Scott
- Old Mother West Wind, by Burgess
- Old Northwest, A Chronicle of the Ohio Valley and Beyond, The, by Frederic Austin Ogg
- Old Peabody Pew, by Kate Douglas Wiggin
- Oldport Days, by Thomas Wentworth Higginson
- Old Roman World, The, by John Lord
- Old Rose and Silver, by Myrtle Reed
- Old Santa Fe Trail, The, by Colonel Henry Inman
- Old Spookses' Pass, by Isabella Valancy Crawford
- Old Stone House, The, by Anne March
- Old Wives' Tale, The, by Arnold Bennett
- Oliver Goldsmith, by Washington Irving
- Oliver Twist, by Charles Dickens
- Oliver Wendell Holmes, by Howells
- Olynthiacs and the Phillippics of Demosthenes, The, by Demosthenes
- Omoo, by Herman Melville
- On a Torn-Away World, by Roy Rockwood
- On Being Human, by Woodrow Wilson
- On Books and The Housing of Them, by William Ewart Gladstone
- Once Aboard The Lugger, by Arthur Stuart-Menteth Hutchinson
- Once Upon A Time In Connecticut, by Caroline Clifford Newton
- On Conducting, by Richard Wagner (translated by Edward Dannreuther)
- One Great Reality, The, by Louisa Clayton
- One of Our Conquerors, by George Meredith
- One of Ours, by Willa Cather
- One Thousand Questions in California Agriculture Answered, by E.J. Wickson
- One Woman, The, by Thomas Dixon
- On Horseback, by Charles Dudley Warner
- On Horsemanship, by Xenophon
- Only True Mother Goose Melodies, The, by Anonymous
- On Nothing & Kindred Subjects, by Hilaire Belloc
- On Our Selection, by Steele Rudd
- On Picket Duty and Other Tales, by Louisa May Alcott
- On Revenues, by Xenophon
- On Some Fossil Remains of Man, by Thomas H. Huxley
- On Something, by H. Belloc
- On the Advisableness of Improving Natural Knowledge, by Thomas H. Huxley
- On The American Contribution, by Winston Churchill
- On the Art of War, by Sun Tzu
- On the Brain, by T. H. Huxley
- On the Brain, by T.H. Huxley
- On the Decay of the Art of Lying, by Twain
- On the Duty of Civil Disobedience, by Henry David Thoreau
- On the Eve, by Ivan Turgenev
- On The Firing Line, by Ray & Fuller
- On the Frontier, by Bret Harte
- On the Improvement of the Understanding, by Baruch Spinoza
- On the Makaloa Mat/Island Tales, by London
- On the Method of Zadig, by Huxley
- On the Origin of Species, by Charles Darwin
- On the Pampas, by G. A. Henty
- ON THE RECEPTION OF THE 'ORIGIN OF SPECIES', by Thomas Henry Huxley
- On the Relations of Man to the Lower Animals, by Thomas H. Huxley
- On The Ruin of Britain, by Gildas Sapiens
- On the Study of Words, by Richard C Trench
- On the Study of Zoology, by Thomas H. Huxley
- On the Track, by Henry Lawson
- On the Trail of Grant and Lee, by Frederick Trevor Hill
- On the Trail of Pontiac, by Edward Stratemeyer
- On War, by Carl von Clausewitz
- Open Air, The, by Richard Jefferies
- Opening a Chestnut Burr, by Edward Payson Roe
- Openings in the Old Trail, by Bret Harte
- O Pioneers!, by Willa Cather
- Opium Habit, The, by Horace B. Day
- Options, by O. Henry
- Orange Fairy Book, The, by Andrew Lang [Editor]
- Orations, by John Quincy Adams
- Orations of Lysias, The, by Lysias
- Ordeal of Richard Feverel, by George Meredith
- Oregon Trail, The, by Francis Parkman, Jr.
- Organic Gardener's Composting, by Steve Solomon
- Organic Syntheses, by James Bryant Conant [Editor]
- Original Narratives of Early American History, by various authors
- Original Short Stories, Maupassant, Entire, by Maupassant
- Original Short Stories, by Maupassant, v10
- Original Short Stories, by Maupassant, v11
- Original Short Stories, by Maupassant, v12
- Original Short Stories, by Maupassant, v13
- Original Short Stories, by Maupassant, v1
- Original Short Stories, by Maupassant, v2
- Original Short Stories, by Maupassant, v3
- Original Short Stories, by Maupassant, v4
- Original Short Stories, by Maupassant, v5
- Original Short Stories, by Maupassant, v6
- Original Short Stories, by Maupassant, v7
- Original Short Stories, by Maupassant, v8
- Original Short Stories, by Maupassant, v9
- Origin and Nature of Emotions, The, by George W. Crile
- Origination of Living Beings, The, by Thomas H. Huxley
- Origin of Species, The, by Huxley
- Origins of Contemporary France, Vol. 1, The, by Hippolyte A. Taine
- Origins of Contemporary France, Vol. 2, The, by Hippolyte A. Taine
- Origins of Contemporary France, Vol. 3, The, by Hippolyte A. Taine
- Origins of Contemporary France, Vol. 4, The, by Hippolyte A. Taine
- Origins of Contemporary France, Vol. 5, The, by Hippolyte A. Taine
- Origins of Contemporary France, Vol. 6, The, by Hippolyte A. Taine
- Orlando Furioso, by Ludovico Ariosto
- Orlando Furioso, by Ludovico Ariosto
- Orpheus in Mayfair and Other Stories and Sketches, by Maurice Baring
- Orthodoxy, by Gilbert K. Chesterton
- Oscar Wilde, His Life and Confessions, by Frank Harris
- Oscar Wilde Miscellaneous, by Oscar Wilde
- Os Lusíadas, by Luís Vaz de Camões
- O. T., A Danish Romance, by Hans Christian Andersen
- Othello, by Wilhelm Hauff
- Other People's Money, by Emile Gaboriau
- Other Things Being Equal, by Emma Wolf
- Otto of the Silver Hand, by Howard Pyle
- Our American Cousin, by Tom Taylor
- Our Androcentric Culture, by C. P. Gilman
- Our Friend John Burroughs, by Clara Barrus
- Our Friend the Charlatan, by George Gissing
- Our Hundred Days in Europe , by Oliver Wendell Holmes
- Our Legal Heritage The first thousand years: 600 - 1600 King AEthelbert - Queen Elizabeth, by S. A. Reilly
- Our Mr. Wrenn, by Sinclair Lewis
- Our Mutual Friend, by Charles Dickens
- Our Nervous Friends, by Robert S. Carroll
- Our Nig, by Harriet E. Wilson
- Our Pilots in the Air, by Captain William B. Perry
- Our Village, by Mary Russell Mitford
- Our War With Spain For Cuba's Freedom, by Trumbull White
- Our World, or, The Slaveholders Daughter, by F. Colburn Adams
- Outdoor Girls at Rainbow Lake, The, by Laura Lee Hope
- Outdoor Girls at Wild Rose Lodge, The, by Laura Lee Hope
- Outdoor Girls in Army Service , The, by Laura Lee Hope
- Outlaw of Torn, The, by Edgar Rice Burroughs
- Outlet, The, by Andy Adams
- Outlines of English and American Literature , by William J. Long
- Out of the Fog, by C. K. Ober
- Out of the Primitive, by Robert Ames Bennet
- Out of the Triangle, by Mary E. Bamford
- Out of Time's Abyss, by Edgar Rice Burroughs
- Out of Time's Abyss, by Edgar Rice Burroughs
- Outpost, by J.G. Austin
- Overland Expedition of The Messrs. Jardine, The, by Messrs. Jardine [Byerley ed.]
- Over Prairie Trails, by Frederick Philip Grove
- Overruled, by George Bernard Shaw
- Over the Border: Acadia, by Eliza Chase
- Over the Sliprails, by Henry Lawson
- Over the Teacups, by Oliver Wendell Holmes
- Over The Top, by Arthur Guy Empey
- Owindia, by Charlotte Selina Bompas
- Oxford[City/Univ], by Andrew Lang
- Oxy-Acetylene Welding and Cutting , by Harold P. Manly
- Ozma of Oz, by L. Frank Baum
