- Olga Romanoff
- Oli, a Star Bridge Too Far
- Oltre il pianeta del vento
- Olympo
- Olympos
- Olympus
- Olympus Union: Jeremy Hunter
- Olá Vizinhos!
- Ombra Meccanica
- Omega
- Omega - The Lost City of Altinova
- Omega Empire
- OMEGA Exile
- Omega Games
- Omega Lab
- Omega Point
- Omega Point: Book 2 of the Deep Fracture Trilogy
- Omega Shadows
- Omega: The Last days of the World
- Omens of Kregen [Dray Prescot #36]
- Omensent: Birth of a Dragon Lord
- Omensent: Revealing the Dragon
- Omensent: Rise of the Shadow Dragons
- Omensent: Wrath of a Dragon God
- Omicidio nel Jet Set
- Omie 17
- Omni
- Omnigalactic: The Eye of Shen'roth
- Omnigalactic: The Fangs of Z'et
- OMNILAND
- Omniphage Invasion
- Omniscience Among Mortals
- On A Cold Wind
- On A Fooler's Errand
- On a Forsaken Land Found
- On a Pale Horse
- On Blue's Waters
- On Distant Shores (Earth Exiles, Book 1)
- On Distant Worlds: The Prologues & Colibri
- On Dragonwings
- On Earth As It Is In Hell
- On Fire
- On Ice
- On Suivi Point
- On the Head of a Pin
- On The Plains of Deception
- On the Trail of Dragons
- On the Wings of Dream
- On to the Asteroid
- On to the Unknown
- On Track (The Old Store Lost Tales: 4)
- On Wings of Magic
- Onamuji El Menor
- Onamuji el Primogénito
- Once a Hero
- Once Broken
- Once Every Year
- Once Upon a Future
- Once Upon a Time in the East
- Once Upon A Time — Volume 1
- Once Upon A Time — Volume 2
- Once Upon A Time — Volume 3
- Once We Were
- Once We Were Human
- One Against Herculum
- One Against the Legion
- One Against the Moon
- One Blood
- One Dark Future
- One Dimension Too Many
- One Foot In Front of the Other
- One Hundred Lives
- One Hundred Shades of Red
- One in 300
- One King's Way
- One King, One Rule
- One Left Behind
- One Man Band
- One Man: No Plan
- One Million Centuries
- One Million Years AD
- One Mind's Eye
- One More Killing
- One of Two
- One Purple Hope!
- One Short Night
- One Shot
- One Small Leap...
- One Star Away
- One Step Behind
- One Tomb Short of a Graveyard
- One Trade Too Many (The Yrden Chronicles Book 3)
- One Tree
- One True Vision
- One Two Punch
- One Way
- One Way To Mars
- One Way To Mars 2
- One Way To Mars 3
- One Winter in Eden
- One World
- One World One People
- Oneiro III: Illusion of Chaos
- Onirica: The Dragonfly's Secret
- Only Begotten Daughter
- Only Human
- Only Sheepdog on the Moon
- Only the Arquebus, by H. Beam Piper
- Only the Inevitable: Books 1 - 5
- Only the Inevitable: Books 6 - 10
- Only the Open
- Only The Truly Damned
- Only Who Is Left: Book I of the Inversion Chronicles
- Onrust
- Onset, Reset, Mindset
- Onurun Bedeli (Krallar ve Büyücüler—3. Kitap)
- Onward to Mars
- Onward! To Europa!
- Onward, Drake!
- Onze jours de siège – suivi d'annexes
- Oogie Finds Love
- Oona, ParaSelene
- Oops, Caught
- Oowans – Curse and Salvation
- Opalescence: The Middle Miocene Play of Color
- Open Arms -I segreti dell'isola- (Nuova edizione)
- Open Distance
- Open Skies
- Opening Atlantis
- Opening Shots
- OPERATION AMAZONAS
- OPERATION ANTARKTIKA
- Operation Arche / Der Krieg der Ketzer
- OPERATION ARKTIS
- Operation Chaos