| Obriart | | 10 |
| Observatoire des inégalités | France | 1 |
| Obsidian Mountain Publishing | Etats-Unis | 23 |
| Occident (CH) | Suisse | 1 |
| Océan | France | 36 |
| Oceano | Mexique | 3 |
| Ocholibros | Chili | 1 |
| OCIM | | 4 |
| ocrée | | 9 |
| Octaedro | Espagne | 4 |
| Octave | Canada | 8 |
| Octobre | France | 47 |
| Octopus | | 14 |
| Octoquill | France | 40 |
| Odé | France | 3 |
| Odepi | France | 1 |
| Odicé | France | 1 |
| Odile Jacob | France | 749 |
| Odin | France | 21 |
| Odyssée | | 2 |
| Odyssey | France | 3 |
| Oeil du Prince | France | 7 |
| Oetinger | Allemagne | 28 |
| Ofelbe | | 87 |
| Off Limits Press | Etats-Unis | 2 |
| Office de publicité | France | 2 |
| Office des Publications Officielles des Communautés Européennes | France | 3 |
| Officina Libraria | Italie | 2 |
| Oficina de Livros | Brésil | 1 |
| Oftomes Publishing | | 1 |
| Ogam-Celticum | France | 1 |
| Ogham | France | 2 |
| Ogmios | | 16 |
| Ogoni | | 3 |
| Oh! | France | 58 |
| Ohdio | Canada | 3 |
| Ohzora Shuppan | Japon | 2 |
| Okama | | 13 |
| Okno | France | 12 |
| Okopix | France | 1 |
| Oktopus | Suisse | 1 |
| Olalde | | 1 |
| Old Mill Press | Etats-Unis | 2 |
| Old Street Publishing | Angleterre | 1 |
| Oldcastle Books | | 4 |
| Oliane | France | 1 |
| Olibris | France | 5 |
| Oliver Heber Books | Etats-Unis | 3 |
| Olivetan | | 5 |
| Olivier Morattel | | 26 |
| Olivier Orban | France | 37 |
| Olivius | France | 4 |
| Olizane | | 10 |
| Olms Zürich | Suisse | 1 |
| Olni | France | 2 |
| Olydri | | 48 |
| Olympe | | 27 |
| Omaké Books | France | 151 |
| Òmara | France | 1 |
| Omblage | France | 8 |
| Ombra | | 1 |
| Ombres | France | 51 |
| Ombres noires | France | 35 |
| Omicron | France | 1 |
| Omnia Veritas | | 11 |
| Omnibus | France | 360 |
| Omnibus / Plon | | 2 |
| Omnibus Press | | 3 |
| Omnific Publishing | Etats-Unis | 14 |
| Omniscience | | 5 |
| Omri Ezrati | France | 2 |
| On ne compte pas pour du beurre | France | 22 |
| On Stream | France | 1 |
| On verra bien | France | 1 |
| Onagro | France | 2 |
| Onapratut | France | 3 |
| Once Upon a Time | Etats-Unis | 2 |
| ONCFS | France | 2 |
| One more chapter | Angleterre | 41 |
| One Planet | Kenya | 1 |
| One Sharp Marriage Publications | Etats-Unis | 1 |
| One World | Etats-Unis | 53 |
| Oneiroi | | 11 |
| Oneworld Publications | | 48 |
| Oni Press | Etats-Unis | 74 |
| Online | France | 1 |
| Onlit | France | 54 |
| Onomatopee | Pays-Bas | 1 |
| Onwe Press | | 2 |
| Onyx | France | 22 |
| Ookilus | France | 11 |
| Ookura shuppan | | 5 |
| Opeasi | | 1 |
| Open Court Publishing | Etats-Unis | 3 |
| Open Letter | Etats-Unis | 4 |
| Open Road | France | 80 |
| Open Strange Doors | France | 2 |
| Opéra | | 9 |
| Ophiledespages | | 6 |
| Ophrys | France | 41 |
