- Optical Orifice of the Beholder
- Optical Osmosis
- Optimizing Acceptance
- Optimizing Evil
- Optimizing Family
- Optimizing Moving
- Optimizing Profit
- Optimizing Swordplay
- Opting In
- Opting Out and Other Departures
- Opting Out: A Story of the Road
- Options of Survival
- Opus 1 - Le Chant de l'Arbre-Mère
- Opus Aria
- Opus Crescendo
- Opus Swarm
- Opzione zero!
- Opération ISKRA
- Opération Lorelei
- Opération mes Sages
- Opération Sabines
- Opération Six Clones
- Opération: Shadowbreaker
- Opętana (Część 12 Wampirzych Dzienników)
- Or
- Or et Nuit
- Or Even Eagle Flew
- Or not to be
- Or What You Will
- Or, Sang, Soleil
- Ora e per sempre
- Ora et labora. Повесть о послушнике Иакове, Святой Инквизиции и таинственных кругах на полях
- Ora i maestri muoiono
- Oracle - Waters Deep (Vol. 6)
- Oracle : Korrigans - Tome 1
- Oracle : Salvertia - Tome 3
- Oracle : Ys city - Tome 2
- Oracle Adored
- Oracle Bones (Bookburners Season 3 Episode 6)
- Oracle Defiant
- Oracle Dream
- Oracle Inception
- Oracle of Nightmares
- Oracle Quest
- Oracle V – Travels Through Other Dimensions
- Oracle's Child
- Oracle's Fire
- Oracle's Legacy: Children of Sun (Book 1)
- Oracle's Legacy: Dawn of Illumination (Book 3)
- Oracle's Legacy: Shadows of Fate (Book 2)
- Oracle's Moon
- Oracle, Clockwork Heart Tale No. 3
- Oracles of Aragret (a Steampunk Novel)
- Oracoli
- Oracular Operations
- Oraculum Dei
- Orad der Schwarze
- Orages en terre de France
- Oraisons
- Oral Argument
- Oran Roy
- Orange Blossom
- Orange Car with Stripes
- Orange Cerlo
- Orange Crush
- Orange Flecks
- Orange Future - Out of Order
- Orange Glory: The Zoomins Must Die
- Orange Image
- Orange Jules ~ Syntax Error
- Orange Storm
- Orange Thief
- Orange World
- Orb and Arrow, Book I: Exploration
- Orb of Light
- Orb of Souls
- Orb Rider: Angel of Shadows
- Orb World of the Dragon
- Orb: A Tale of Alien Pregnancy Part 2: Another Day, Another Belly, and a Threesome
- Orbat
- Orbe, cetro, trono
- Orbias
- Orbias - o Demónio Branco
- Orbis
- Orbis Onirika Primo Volume
- Orbit Unlimited
- Orbital Cloud
- Orbital Extraction
- Orbital Havoc
- Orbital Lily
- Orbital Maneuvers
- Orbital Transmissions
- Orbitale Balance
- Orbite Strane 1: La Bambina Sperduta
- Orbiting Sins
- Orbs
- Orbs of Avalon: Tales of Urban Magick and Horror
- Orbs of Azure
- ORBS: The Complete 3-Book Science Fiction Series
- Orc Eroica, Vol. 1 (light novel)
- Orc Eroica, Vol. 2 (light novel)
- Orc Haven
- Orc In Winter
- Orc Warfare
- Orchid & The Great Nudini
- Orchid in the Void
- Orchid Nights
- Orchidaceous: Exsanguinate Book 1
- Orchideen
- Orchéron
- Orconomics
- Orcs
- Orcs Bad Blood I
- Orcs Bad Blood II
- Orcs Bad Blood III
- Orcs War-Fighting Manual
- Orcslayer
- Orcus Cave
- Orcus Gammeus
- Orcus Unchained
- Ordained Chronicles
- Ordeal of the Dragon
- Ordelen
- Ordem
- Ordem vermelha
- Order 66