- On the eve: A novelAuthor: Ivan Sergeevich Turgenev, Constance GarnettReleased on Project Gutenberg: November 1, 2004
- On the Evolution of Language: First Annual Report of the Bureau of Ethnology to the Secretary of the Smithsonian Institution, 1879-80, Government Printing Office, Washington, 1881, pages 1-16Author: John Wesley PowellReleased on Project Gutenberg: July 13, 2006
- On the Firing LineAuthor: Anna Chapin Ray, Fuller, Hamilton BrockReleased on Project Gutenberg: January 1, 2003
- On the Firing Line in EducationAuthor: Ladd, Adoniram JudsonReleased on Project Gutenberg: June 7, 2007
- On the Fringe of the Great FightAuthor: George Gallie NasmithReleased on Project Gutenberg: November 20, 2006
- On the FrontierAuthor: Bret HarteReleased on Project Gutenberg: May 18, 2006
- On the Genesis of SpeciesAuthor: St. George Jackson MivartReleased on Project Gutenberg: March 14, 2007
- On the Heels of De WetAuthor: Lionel JamesReleased on Project Gutenberg: January 19, 2007
- On the Improvement of the UnderstandingAuthor: Benedictus de Spinoza, R. H. M. (Robert Harvey Monro) ElwesReleased on Project Gutenberg: August 1, 1997
- On the Indian Sect of the JainasAuthor: Georg Bühler, James BurgessReleased on Project Gutenberg: April 1, 2004
- On the Indian Trail: Stories of Missionary Work among Cree and Salteaux IndiansAuthor: Egerton Ryerson YoungReleased on Project Gutenberg: October 31, 2007
- On the Irrawaddy: A Story of the First Burmese WarAuthor: G. A. (George Alfred) Henty, William Heysham OverendReleased on Project Gutenberg: April 27, 2007
- On the King's Service: Inward Glimpses of Men at ArmsAuthor: Logan, InnesReleased on Project Gutenberg: November 3, 2005
- On the Method of Zadig: Essay #1 from "Science and Hebrew Tradition"Author: Thomas Henry HuxleyReleased on Project Gutenberg: May 1, 2001
- On the Nature of ThingsAuthor: Lucretius Carus, Titus, 95? BCE-50? BCE, William Ellery LeonardReleased on Project Gutenberg: January 1, 1997
- On the Old Road Vol. 1 (of 2): A Collection of Miscellaneous Essays and Articles on Art and LiteratureAuthor: John RuskinReleased on Project Gutenberg: June 2, 2008
- On the Old Road, Vol. 2 (of 2): A Collection of Miscellaneous Essays and Articles on Art and LiteratureAuthor: John RuskinReleased on Project Gutenberg: April 30, 2007
- On the Origin of Species by Means of Natural Selection: or the Preservation of Favoured Races in the Struggle for Life. (2nd edition)Author: Charles DarwinReleased on Project Gutenberg: September 25, 2007
- On the Origin of Species By Means of Natural Selection: Or, the Preservation of Favoured Races in the Struggle for LifeAuthor: Charles DarwinReleased on Project Gutenberg: March 1, 1998
- On the Pampas; Or, The Young SettlersAuthor: G. A. (George Alfred) HentyReleased on Project Gutenberg: September 1, 2004
- On the portraits of English authors on gardeningAuthor: Felton, SamuelReleased on Project Gutenberg: June 13, 2008
- On the Prospects of Christianity: Bernard Shaw's Preface to Androcles and the LionAuthor: Bernard ShawReleased on Project Gutenberg: May 1, 2003
- On the Reception of the 'Origin of Species'Author: Thomas Henry HuxleyReleased on Project Gutenberg: February 1, 2000
- On the Relations of Man to the Lower AnimalsAuthor: Thomas Henry HuxleyReleased on Project Gutenberg: November 1, 2001
- On the Ruin of BritainAuthor: Gildas, J. A. (John Allen) GilesReleased on Project Gutenberg: November 1, 1999
- On the Seashore: Cassell's "Eyes and No Eyes" Series, Book VIIAuthor: Smith, R. CadwalladerReleased on Project Gutenberg: December 1, 2003
- On the Significance of Science and ArtAuthor: Tolstoy, Leo, graf, Isabel Florence HapgoodReleased on Project Gutenberg: January 1, 2003
- On the Spanish Main; Or, Some English forays on the Isthmus of DarienAuthor: John MasefieldReleased on Project Gutenberg: September 28, 2006
- On the StairsAuthor: Henry Blake FullerReleased on Project Gutenberg: May 26, 2007
- On the Study of WordsAuthor: Richard Chenevix TrenchReleased on Project Gutenberg: September 1, 2004
- On the Study of ZoologyAuthor: Thomas Henry HuxleyReleased on Project Gutenberg: November 1, 2001
- On the SublimeAuthor: Longinus, active 1st century, Andrew Lang, Havell, H. L. (Herbert Lord), -Released on Project Gutenberg: March 10, 2006
- On the TrackAuthor: Henry LawsonReleased on Project Gutenberg: March 1, 1998
- On the Trail of Grant and LeeAuthor: Frederick Trevor HillReleased on Project Gutenberg: May 1, 2003
- On the Trail of Pontiac; Or, The Pioneer Boys of the OhioAuthor: Edward StratemeyerReleased on Project Gutenberg: September 1, 2004
- On the Trail of the Space PiratesAuthor: Rockwell, Carey, Willy Ley, Glanzman, Louis [Illustrator]Released on Project Gutenberg: April 15, 2007
- On the Trail: An Outdoor Book for GirlsAuthor: Lina Beard, Adelia B. (Adelia Belle) BeardReleased on Project Gutenberg: June 7, 2006
- On the Tree TopAuthor: Clara Doty Bates, Curtis, Jessie [Illustrator], Frank T. MerrillReleased on Project Gutenberg: February 6, 2008
- On the Vice of Novel Reading.: Being a brief in appeal, pointing out errors of the lower tribunal.Author: Young Ewing AllisonReleased on Project Gutenberg: February 27, 2008
- On WarAuthor: Carl von Clausewitz, Graham, J. J. (James John) [Translator]Released on Project Gutenberg: February 26, 2006
- On With TorchyAuthor: Sewell Ford, Lincoln, F. Foster (Frederic Foster) [Illustrator]Released on Project Gutenberg: December 13, 2005
- Once a weekAuthor: A. A. (Alan Alexander) MilneReleased on Project Gutenberg: January 15, 2008
- Once Aboard the Lugger-- The History of George and his MaryAuthor: A. S. M. (Arthur Stuart-Menteth) HutchinsonReleased on Project Gutenberg: September 1, 2004
- Once Upon A TimeAuthor: Richard Harding DavisReleased on Project Gutenberg: October 19, 2005
- Once Upon a Time in ConnecticutAuthor: Newton, Caroline Clifford, -Released on Project Gutenberg: October 1, 2004
- One BasketAuthor: Edna FerberReleased on Project Gutenberg: April 1, 1996
- One Common FaithAuthor: Bahá'í International CommunityReleased on Project Gutenberg: September 17, 2006
- One Day: A sequel to 'Three Weeks'Author: Elinor GlynReleased on Project Gutenberg: October 18, 2004
- One Day At ArleAuthor: Frances Hodgson BurnettReleased on Project Gutenberg: November 4, 2007
- One Day More: A Play In One ActAuthor: Joseph ConradReleased on Project Gutenberg: January 29, 2006
- One Day's Courtship, and The Heralds of FameAuthor: Robert BarrReleased on Project Gutenberg: November 1, 2005
- One Hundred Best Books: With Commentary and an Essay on Books and ReadingAuthor: John Cowper PowysReleased on Project Gutenberg: July 15, 2004
- One Hundred Merrie And Delightsome Stories: Right Pleasaunte To Relate In All Goodly Companie By Way Of Joyance And JollityAuthor: Robert B. (Robert Bruce) Douglas, La Sale, Antoine de, Léon LebèqueReleased on Project Gutenberg: June 13, 2006
- One Man in His TimeAuthor: Ellen Anderson Gholson GlasgowReleased on Project Gutenberg: April 11, 2005
- One Man's Initiation—1917Author: John Dos PassosReleased on Project Gutenberg: January 7, 2008
- One of Life's SlavesAuthor: Jonas Lie, Muir, Jessie [Translator]Released on Project Gutenberg: May 18, 2005
- One of Our Conquerors — CompleteAuthor: George MeredithReleased on Project Gutenberg: November 5, 2004
- One of Our Conquerors — Volume 1Author: George MeredithReleased on Project Gutenberg: September 1, 2003
- One of Our Conquerors — Volume 2Author: George MeredithReleased on Project Gutenberg: September 1, 2003
- One of Our Conquerors — Volume 3Author: George MeredithReleased on Project Gutenberg: September 1, 2003