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Flann O'Brien
The Dalkey Archive UK US
The Third Policeman UK US

Gerry O'Brien
Series: The Burough
Cleaning Up UK US
Planting Out UK US

Robert C. O'Brien
Series: Rats of NIMH
Mrs. Frisby and the Rats of NIMH UK US

Nick O'Donohoe
Series: Crossroads
The Magic and the Healing US
Under the Healing Sign UK US
The Healing of Crossroads UK US
Series: Gnomewrench
The Gnomewrench in the Dwarfworks UK US
The Gnomewrench in the Peopleworks UK US

Seán O'Faoláin
And Again? US

Tim O'Laughlin
Phoenix Fire US

Patrick O'Leary
The Gift UK US

Anthony O'Neill
Sheherazade UK

Darren O'Shaughnessy
Series: The City
Ayuamarca, Procession of the Dead UK
Hell's Horizon UK

Pat O'Shea
The Hounds of the Morrigan UK US
Finn MacCool and the Small Men of Deeds US

Mel Odom
Series: Forgotten Realms: The Threat from the Sea
Rising Tide UK US
Under Fallen Stars UK US
The Sea Devil's Eye UK US
Series: Forgotten Realms: Lost Empires
The Lost Library of Cormanthyr US
Series: Forgotten Realms: The Cities
The Jewel of Turmish UK US
Series: Diablo
The Black Road with Charles L. Grant UK US
Series: Might and Magic
The Sea of Mist UK US
Series
The Rover UK US
The Destruction of the Book UK US
Lord of the Libraries UK US

Andrew J. Offutt
Series: Conan
The Sword of Skelos US
Conan and the Sorcerer UK US
Conan the Mercenary US
Series: Cormac Mac Art
The Mists of Doom US
The Undying Wizard US
When Dead Birds Fly with Keith Taylor US
The Tower of Death with Keith Taylor US
The Sword of the Gael with Robert E. Howard US
The Sign of the Moonbow US
Series: Thieves World
Shadowspawn US
Deathknight US
The Shadow of Sorcery US
Series: War of the Gods on Earth
The Iron Lords US
Shadows Out of Hell US
The Lady of the Snowmist US
Series: War of the Wizards
Demon in the Mirror with Richard K. Lyon US
The Eye of Sarsis with Richard K. Lyon US
Web of the Spider with Richard K. Lyon US
My Lord Barbarian US
Deathknight US

Ian Ogilvy
Series
Measle and the Wrathmonk UK US
Measle and the Dragodon UK US
Measle and the Mallockee UK US

Noriko Ogiwara
Dragon Sword and Wind Child US

Nnedi Okorafor-Mbachu
Zahrah the Windseker UK US

Ben Okri
Series
The Famished Road UK US
Songs of Enchantment UK US
Infinite Riches UK US

Jenny Oldfield
Series: Dream Seeker
Silver Cloud UK

June Oldham
The Raven Waits UK

Gloria Oliver
In Service of Samurai UK US
Vassal of El UK US

Jana G. Oliver
Series: DragonFire
The Circle of the Swan UK US
The Summoning Stone UK US

Lance Olsen
Live from Earth US

Kenneth Oppel
Series
Silverwing UK US
Sunwing UK US
Firewing UK US
Series
Airborn UK US
Skybreaker UK US

Shulamith Oppenheim
The World Invisible US

Kerry L. Orchard
The Thoughtmaster's Conduit

Rebecca Ore
Slow Funeral US

A. Orr
Series: World in Amber
The World in Amber US
In the Ice King's Palace US

Joe Orton
Head to Toe UK US

Victor Osborne
Moondream US

Juliene Osborne-McKnight
Daughter of Ireland UK US
Bright Sword of Ireland UK US

Ovid
Metamorphoses UK US

Robin D. Owens
Series: The Summonings
Guardian of Honor UK US
Sorceress of Faith UK US
Protector of the Flight UK US
Keepers of the Flame UK US

Helen Oyeyemi
The Icarus Girl UK US

Cynthia Ozick
The Puttermesser Papers UK US