Books about Adventure stories (sorted alphabetically)
- O'Ruddy: A Romance Stephen Crane and Robert Barr
- Oscar in Africa Harry Castlemon
- Oswald Bastable and Others E. Nesbit
- Our Fellows; Or, Skirmishes with the Swamp Dragoons Harry Castlemon
- Outcasts of Solar III Robert Emmett McDowell
- Outdoor Chums in the Big Woods; Or, Rival Hunters of Lumber Run Quincy Allen
- Outdoor Chums in the Forest; Or, Laying the Ghost of Oak Ridge Quincy Allen
- Outdoor Girls Around the Campfire; or, The Old Maid of the Mountains Laura Lee Hope
- Outdoor Girls at Bluff Point; Or a Wreck and a Rescue Laura Lee Hope
- Outdoor Girls at the Hostess House; Or, Doing Their Best for the Soldiers Laura Lee Hope
- Outdoor Girls in a Motor Car; Or, The Haunted Mansion of Shadow Valley Laura Lee Hope
- Outdoor Girls in Army Service; Or, Doing Their Bit for the Soldier Boys Laura Lee Hope
- Outdoor Girls in a Winter Camp Laura Lee Hope
- Outdoor Girls of Deepdale; Or, camping and tramping for fun and health Laura Lee Hope
- Outlaw Jack; or, the mountain devil Jos. E. Badger
- outlaw's pledge; or, The raid on the old stockade Spencer Dair
- Out of Russia Crittenden Marriott
- Out of the Iron Womb! Poul Anderson
- Outside the universe Edmond Hamilton
- Outward Bound Or, Young America Afloat: A Story of Travel and Adventure Oliver Optic
- Overlord of Colony Eight Robert Silverberg
- Owen Clancy's Run of Luck; or, The Motor Wizard in the Garage Burt L. Standish
- Owen Hartley; or, Ups and Downs: A Tale of Land and Sea William Henry Giles Kingston
- Pacha of Many Tales Frederick Marryat
- Pacha of Many Tales Frederick Marryat