Publishers – O
- O Books
- O'Brien Press
- O'Reilly Media
- Oak Archives
- Oak Tree Books
- Oak Tree Press
- Oberon Books
- Octopus Publishing Group
- Officina Libraria
- Ohio State University Press
- Ohio University Press
- Old Street Publishing
- Oldcastle Books
- Olympia Publishers
- Omnibus Press
- One Love Books
- One Time Publishing
- Oneworld Publications
- Oni Press
- Onomatopee
- Open Court Publishing
- Open Road
- Open Road Distribution
- Open Universe
- Open University Press
- OR Books
- Orange Hippos!
- Orange Mosquito
- Oratia Books
- Oregon State University
- Origin
- Original Falcon Press
- Orion Books
- Orion Publishing Co
- Oro Editions
- Orphans Press
- Ossian Publications
- Otago University Press
- Other Press
- OUP Archive
- Our House Publishing
- Out of the Ark Music
- Outline Press
- Overlook Press
- Overture Publishing
- Ovolo Books
- Ovolo Publishing
- Oxford University Press