- Operation Oracle: Merkiaarri Wars 3
- Operation Outer Space
- Operation Phoenix
- Operation Phoenix Book 1: We're Not Alone
- Operation Phoenix Book 2: Bohina
- Operation Phoenix Book 3: Imperial Guard
- Operation Phoenix Book 4: Invasion
- Operation Razmer
- Operation Rubber Ducky
- Operation Sasquatch: The Piñon Paradox
- Operation Sleeping Dragon
- Operation Starseed
- Operation Sunset
- Operation Terror (Serapis Classics)
- Operation Vermont
- Operation: Endgame
- Operazione drago
- Opowiadania najlepsze
- Opowiadania zebrane, tom 1
- Opowiadania zebrane, Tom 2
- Opportunity Calls Chinavare's Find Book 1
- Opportunity Calls: Chinavare's Find Book One
- Oppose
- Opposite Land
- Opposition
- Options of Survival
- Opus 1 - Le Chant de l'Arbre-Mère
- Opus Swarm
- Opzione zero!
- Oracle's Child
- Oracle, Clockwork Heart Tale No. 3
- Oracles of Aragret (a Steampunk Novel)
- Oracular Operations
- Oracular Operations
- Orange City
- Orange Flecks
- Orb: A Tale of Alien Pregnancy Part 2: Another Day, Another Belly, and a Threesome
- Orbat
- Orbit
- Orbital Havoc
- Orbital Lily
- Orbite Strane 1: La Bambina Sperduta
- Order 66
- Order of Rings: Uncommon Synths
- Order of the Fallen
- Order of the Lily
- Order of the Lily, the Complete Series
- Order of the Phoenix
- Order of the Seers
- Order of the Seers: The Last Seer
- Order of the Seers: The Red Order
- Orders to Leave
- Ordnance
- Orfanato
- Organická oprátka
- Orgoglio e sacrificio
- Orient Fevre
- Orientation (A Dystopian Fantasy)
- Origin of Pietas
- Origin of Prometheus
- Origin Phase
- Origin Phase Cycle Box Set
- Original
- Original Syn
- Origine perdue
- Origines
- Origins
- Origins: Curve of Humanity Book One
- Orillion Prime
- Orion
- Orion Among the Stars
- Orion and King Arthur
- Orion Arm
- Orion Four
- Orion Gambit
- Orion Tilt
- Orlo's Orbits
- Orltron V: The Syn of Paruti
- ORN'S MANUAL TO HUMAN TELEPORTATION
- Oroboro Island
- Orologeria
- Orphan's Legacy
- Orphan's Test
- Orphan: Desecration
- Orphaned Voices
- Orphans of the Celestial Sea, Episode 3: Mysterious Cargo
- Orphans of War
- Orta
- Oryx and Crake
- Oryx y Crake (Trilogía de MaddAddam 1)
- Os dias em que Rúbia viveu no futuro
- Os Imperios de Vidro: As Aventuras do Pipen, O Aristocrata
- Osa
- Osada
- Osama the Gun
- Oscillation Rising
- Oscuridad