| 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 | 36 |
| Olivius | France | 4 |
| Olizane | | 10 |
| Olms Zürich | Suisse | 1 |
| Olni | France | 2 |
| Olydri | | 48 |
| Olympe | | 25 |
| Omaké Books | France | 149 |
| Òmara | France | 0 |
| Omblage | France | 8 |
| Ombra | | 1 |
| Ombres | France | 51 |
| Ombres noires | France | 35 |
| Omicron | France | 1 |
| Omnia Veritas | | 10 |
| Omnibus | France | 358 |
| 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 | 21 |
| On Stream | France | 1 |
| On verra bien | France | 1 |
| Onagro | France | 2 |
| Onapratut | France | 3 |
| Once Upon a Time | Etats-Unis | 2 |
| One more chapter | Angleterre | 40 |
| 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 | 72 |
| Online | France | 1 |
| Onlit | France | 54 |
| Onomatopee | Pays-Bas | 0 |
| Onwe Press | | 2 |
| Onyx | France | 22 |
| Ookilus | France | 9 |
| Ookura shuppan | | 5 |
| Opeasi | | 1 |
| Open Court Publishing | Etats-Unis | 1 |
| Open Letter | Etats-Unis | 4 |
| Open Road | France | 78 |
| Open Strange Doors | France | 2 |
| Opéra | | 9 |
| Ophiledespages | | 6 |
| Ophrys | France | 41 |
| Opta | France | 189 |
| Optic | France | 1 |
| OQO | Espagne | 17 |
| Or Books | Angleterre | 1 |
| Orange Verte | France | 1 |
| Orb Books | Etats-Unis | 15 |
| Orban | France | 2 |
| Orbit | Angleterre | 873 |
| Orca Book Publishers | France | 9 |
| Orchard Books | Angleterre | 51 |
| Ordalaget Bokförlag | Suède | 13 |
| Orenda books | Angleterre | 19 |
| Orep | France | 44 |
| Organic | | 4 |
| Organisation des Nations Unies (ONU) | | 3 |
| Orients | | 4 |
| Original Books | France | 23 |
| Original Watts | France | 5 |
| Originale | | 1 |
| Orion Books | Angleterre | 645 |
| Oris | France | 2 |
| Orizons | France | 9 |
| Orphée | France | 6 |
| Orphée / La Différence | | 10 |
| Orphelya | France | 3 |
| Orphie | France | 47 |
