- The Ontario High School ReaderAuthor: Marty, Aletta E.Released on Project Gutenberg: September 28, 2007
- The Ontario Readers: Fourth BookAuthor: Ontario. Department of EducationReleased on Project Gutenberg: June 27, 2006
- The Ontario Readers: The High School Reader, 1886Author: Ontario. Department of EducationReleased on Project Gutenberg: November 27, 2006
- The Ontario Readers: Third BookAuthor: Ontario. Department of EducationReleased on Project Gutenberg: June 12, 2006
- The Opal SerpentAuthor: Fergus HumeReleased on Project Gutenberg: March 6, 2008
- The Open AirAuthor: Richard JefferiesReleased on Project Gutenberg: November 1, 2004
- The Open Door, and the Portrait.: Stories of the Seen and the Unseen.Author: Mrs. (Margaret) OliphantReleased on Project Gutenberg: November 1, 2003
- The Open Secret of IrelandAuthor: Tom KettleReleased on Project Gutenberg: March 7, 2005
- The Opened Shutters: A NovelAuthor: Clara Louise Burnham, Harrison FisherReleased on Project Gutenberg: July 2, 2008
- The Opera: A Sketch of the Development of Opera. With full Descriptions of all Works in the Modern Repertory.Author: R. A. (Richard Alexander) Streatfeild, J. A. (John Alexander) Fuller-MaitlandReleased on Project Gutenberg: July 9, 2005
- The Opinions of a PhilosopherAuthor: Robert GrantReleased on Project Gutenberg: October 9, 2006
- The Opium HabitAuthor: Horace B. DayReleased on Project Gutenberg: January 1, 2005
- The Orange Fairy BookAuthor: Andrew LangReleased on Project Gutenberg: January 1, 2002
- The Orange-Yellow DiamondAuthor: J. S. (Joseph Smith) FletcherReleased on Project Gutenberg: November 1, 2005
- The Orations of LysiasAuthor: LysiasReleased on Project Gutenberg: March 1, 2004
- The Orations of Marcus Tullius Cicero, Volume 4Author: Cicero, Marcus Tullius, Charles Duke YongeReleased on Project Gutenberg: February 1, 2004
- The Ordeal of Richard Feverel — CompleteAuthor: George MeredithReleased on Project Gutenberg: November 2, 2004
- The Ordeal of Richard Feverel — Volume 1Author: George MeredithReleased on Project Gutenberg: September 1, 2003
- The Ordeal of Richard Feverel — Volume 2Author: George MeredithReleased on Project Gutenberg: September 1, 2003
- The Ordeal of Richard Feverel — Volume 3Author: George MeredithReleased on Project Gutenberg: September 1, 2003
- The Ordeal of Richard Feverel — Volume 4Author: George MeredithReleased on Project Gutenberg: September 1, 2003
- The Ordeal of Richard Feverel — Volume 5Author: George MeredithReleased on Project Gutenberg: September 1, 2003
- The Ordeal of Richard Feverel — Volume 6Author: George MeredithReleased on Project Gutenberg: September 1, 2003
- The Ordeal: A Mountain Romance of TennesseeAuthor: Charles Egbert Craddock, Douglas DuerReleased on Project Gutenberg: November 13, 2006
- The Ordinance of CovenantingAuthor: John CunninghamReleased on Project Gutenberg: May 6, 2008
- The Oregon Trail: Sketches of Prairie and Rocky-Mountain LifeAuthor: Francis ParkmanReleased on Project Gutenberg: April 27, 2006
- The Organization of the Congregation in the Early Lutheran Churches in AmericaAuthor: B. M. (Beale Melanchthon) SchmuckerReleased on Project Gutenberg: October 1, 2006
- The Oriental Religions in Roman PaganismAuthor: Franz CumontReleased on Project Gutenberg: August 1, 2007
- The oriental story bookAuthor: Wilhelm Hauff, John William Orr, G. P. (George Payn) QuackenbosReleased on Project Gutenberg: February 13, 2008
- The Origin and Deeds of the GothsAuthor: Jordanes, active 6th century, Charles Christopher MierowReleased on Project Gutenberg: January 26, 2005
- The Origin and Nature of the Emotions; Miscellaneous PapersAuthor: George Washington CrileReleased on Project Gutenberg: March 1, 1998
- The Origin and Permanent Value of the Old TestamentAuthor: Charles Foster KentReleased on Project Gutenberg: July 1, 2005
- The Origin of Species: From 'The Westminster Review', April 1860Author: Thomas Henry HuxleyReleased on Project Gutenberg: November 1, 2001
- The Origin of Species by Means of Natural Selection: Or, the Preservation of Favoured Races in the Struggle for Life, 6th EditionAuthor: Charles DarwinReleased on Project Gutenberg: December 1, 1999
- The Original Fables of La Fontaine: Rendered into English Prose by Fredk. Colin TilneyAuthor: Jean de La Fontaine, F. C. (Frederick Colin) TilneyReleased on Project Gutenberg: May 30, 2005
- The Origins of Contemporary France, Complete Table of ContentsAuthor: Hippolyte Taine, Rom, Svend [Annotator], David Widger, John DurandReleased on Project Gutenberg: June 22, 2008
- The Other GirlsAuthor: A. D. T. (Adeline Dutton Train) WhitneyReleased on Project Gutenberg: July 19, 2005
- The Other Side of the DoorAuthor: Lucia Chamberlain, Herman PfeiferReleased on Project Gutenberg: June 8, 2008
- The Otterbein Hymnal: For Use in Public and Social WorshipAuthor: Edmund S. (Edmund Simon) LorenzReleased on Project Gutenberg: August 6, 2005
- The OutcastsAuthor: William Alexander Fraser, Arthur HemingReleased on Project Gutenberg: September 26, 2006
- The OutcryAuthor: Henry JamesReleased on Project Gutenberg: June 29, 2007
- The Outdoor Chums After Big Game; Or, Perilous Adventures in the WildernessAuthor: Allen, QuincyReleased on Project Gutenberg: February 27, 2005
- The Outdoor Chums at Cabin Point; Or, The Golden Cup MysteryAuthor: Allen, QuincyReleased on Project Gutenberg: November 9, 2006
- The Outdoor Chums on the Gulf; Or, Rescuing the Lost BalloonistsAuthor: Allen, QuincyReleased on Project Gutenberg: November 23, 2004
- The Outdoor Chums; Or, The First Tour of the Rod, Gun and Camera ClubAuthor: Allen, QuincyReleased on Project Gutenberg: November 1, 2003
- The Outdoor Girls at Bluff Point; Or a Wreck and a RescueAuthor: Hope, Laura LeeReleased on Project Gutenberg: January 10, 2007
- The Outdoor Girls at Ocean View; Or, The Box That Was Found in the SandAuthor: Hope, Laura LeeReleased on Project Gutenberg: September 16, 2006
- The Outdoor Girls at Rainbow Lake; Or, The Stirring Cruise of the Motor Boat GemAuthor: Hope, Laura LeeReleased on Project Gutenberg: January 1, 2004
- The Outdoor Girls at the Hostess House; Or, Doing Their Best for the SoldiersAuthor: Hope, Laura LeeReleased on Project Gutenberg: November 24, 2004
- The Outdoor Girls at Wild Rose Lodge; Or, The Hermit of Moonlight FallsAuthor: Hope, Laura LeeReleased on Project Gutenberg: January 1, 2004
- The Outdoor Girls at Wild Rose Lodge; or, the Hermit of Moonlight FallsAuthor: Hope, Laura LeeReleased on Project Gutenberg: June 1, 2005
- The Outdoor Girls in a Motor Car; Or, The Haunted Mansion of Shadow ValleyAuthor: Hope, Laura LeeReleased on Project Gutenberg: October 17, 2006
- The Outdoor Girls in a Winter Camp: Or, Glorious Days on Skates and Ice BoatsAuthor: Hope, Laura LeeReleased on Project Gutenberg: October 23, 2006
- The Outdoor Girls in Army Service; Or, Doing Their Bit for the Soldier BoysAuthor: Hope, Laura LeeReleased on Project Gutenberg: February 1, 2005
- The Outdoor Girls in Florida; Or, Wintering in the Sunny SouthAuthor: Hope, Laura LeeReleased on Project Gutenberg: September 17, 2006
- The Outdoor Girls in the Saddle; Or, The Girl Miner of Gold RunAuthor: Hope, Laura LeeReleased on Project Gutenberg: September 18, 2006
- The Outdoor Girls of Deepdale; Or, camping and tramping for fun and healthAuthor: Hope, Laura LeeReleased on Project Gutenberg: December 1, 2003
- The Outdoor Girls on Pine Island; Or, A Cave and What It ContainedAuthor: Hope, Laura LeeReleased on Project Gutenberg: September 16, 2006
- The outlaw of TornAuthor: Edgar Rice BurroughsReleased on Project Gutenberg: July 11, 2008
- The OutletAuthor: Andy AdamsReleased on Project Gutenberg: December 1, 1999