- O entrelaçamento electroquântico de que são feitas as lendas (Comandante Serralves)
- O Fim do Mundo
- O Incrível Congresso de Astrobiologia
- O interrogador
- O jardim cosmogônico
- O meu irmão é um marginal - Livro 2 Liberdade
- O Portal da Pirâmide
- O Primmeiro Contacto
- O Retrato Cinza de Dorian
- O xenólogo de Sírius
- O'Mally's Station
- Oak Ridge
- Oasi
- Oasis Tales of the Conjuror: and other stories
- Obiekt 271
- Oblio
- Oblivion
- Oblivionlore Volume I
- Obsession
- Obsessions: An Anthology of Original Fiction
- Obsidian Worlds: 11 Mind-Bending Sci-Fi Shorts
- Occupied Earth
- Occupy Stinking America
- OCEANS: The Anthology
- Octopocalypse
- Odd Numbers
- Odd, Strange and Curious
- Ode to Joy
- Odiabili resti
- Odin's Eye
- Odjuren Hela bokserien
- Odregan #1
- Odregan #2
- Odyssee der Sternfahrer: Science Fiction Sammelband
- Odyssee in die Unendlichkeit: Raumschiff Rubikon Band 1-8: Science Fiction Abenteuer Paket
- Odyssées chimériques
- Oeps!
- Of Dust and Sand
- Of Gods and Globes
- Of Love, Free Will, and Gray Squirrels on a Summer Evening
- Of Myth And Monster
- Of No Return
- Of Shallow Graves and Contacts Lost
- Of Sporks, Overlords, and Moon Worms
- Off Limits
- Off the Beaten Path
- Off the Face of the Earth
- Ogni nave deve incontrare le sue onde
- Oh No! Zombies!
- Oil to Ashes 1, "Picnic" (Linc Freemore Apocalyptic Thriller Series)
- Oklahoma Salvage
- Old Dwarf
- Old Friends Are the Best
- Old Man's Time
- Old Mars
- Old Odd Ends
- Old Tales Retold
- Old Venus
- Old Wounds
- Oliver Oxford's for God and Country
- Oliver: A Wendy Miracle Short Story
- Oltre il reale. Il meglio della fantascienza indipendente 2020
- Ombak Volume One
- Ombre di plastica
- Omega and Alpha & Rest My Brothers
- Omega Shadows
- Omnidirectional
- Omnimax II
- On a little street in Singapore
- On Ice
- On the Eyeball Floor
- On the Great Wall of Texas
- On the Many Uses of Cedar
- On the Outskirts of Saturn
- On The Plains of Deception
- On the Rocks
- On Trial
- Onamuji El Menor
- Onamuji el Primogénito
- Once a Greech
- Once a Hero, Always a Hero
- Once Upon a Dead Gull
- Once Upon a Fact
- Once Upon a Future
- Once Upon a Gas Tank
- Once Upon A Parallel Time
- Once Upon A Planet
- Once Upon a Time in Gravity City
- One
- One and Wonder
- One Awake in All the World
- One Billion Faces
- One Cog Dreaming
- One Cold December Night
- One Drop In Time
- One For The Road
- One Frame At A Time
- One Heartbeat Past Normal
- One Hundred, Ten, and One
- One Last Goodbye
- One Last Look: a postapocalyptic YA short story (Bunker Story #1)
- One Last Xanax
- One Man's Poison and five more stories
- One Million A.D.
- One Minute to Midnight
- One Night at Sharon's
- One Night in London
- One Out of Ten
- One Plus One Makes Me
- One Purple Hope!
- One Small Step, an Anthology of Discoveries
- One Smaller Step
- One Small Step, an Anthology of Discoveries
- One Smaller Step
- One Step Beyond: An Anthology of Science Fiction and Fantasy
- One Step from Earth
- One Thousand Eyes
- One Year of Instants
- One Year of Instants (2018)
- One-Shot
- Oneness
- Only Disconnect
- Only Human
- Only One Winner
- Only the Arquebus, by H. Beam Piper
- Onslaught from Rigel
- Onward, Drake!
- Onyx (A YA SciFi Retelling of Othello)
- Oomphel in the Sky
- Oops An Alien Got Me Pregnant!
- Oops!
- Oops!!!
- Oort
- Open Ended Threat: A Cave Creek Anthology
- Open Wide and Say Ocule: A Broken Spaceship Sci Fi Short Story
- Operation R.S.V.P.
- Operation Rubber Ducky
- Operation Terror
- Operazione Pettirosso
- Ophal
- Ophelia In My Arms
- Ophelia Omarmd
- Ophid Dei
- Opowieści o pilocie Pirxie
- Opting Out and Other Departures
- Opting Out: A Story of the Road
- Opus X Series Boxed Set One
- Opzione zero!
- Ora i maestri muoiono
- Oracular Operations
- Orange Jules ~ Syntax Error
- Orbital Maneuvers
- Orbitale Balance
- ORBS: The Complete 3-Book Science Fiction Series
- Oregon Elsewise: Eight short stories of an Oregon that never was
- Orgoglio e sacrificio
- Orienteer
- Origin
- Origin of Species
- Origins
- Origins of the New Breed of Super Hero
- Origins of the New Breed of Super Villain
- Orion Effect
- Orion, Femspec Issue 6.2
- Orion: Suresh Adventure
- Ornery Oldtimers
- Orotund: Collected Short Stories Volume Two