Publishers
- O.P.C. 1
- O.R. Reisland 1
- O'Reilly 1
- Oak Knoll Press 3
- Oaklands 1
- Oakmagic Publications 2
- Oakridge Historical Research Group 2
- Obscurus Books 1
- Obv 1
- Ocean Boos Pvt. Ltd 1
- Octopus 2
- Octopus Books Ltd. 4
- Odhams Books 2
- Odhams Press 2
- Odhams Press Limited 5
- Odhams Press Ltd. 4
- Oesch Verlag 1
- Office For Official Publications Of The European Communities 1
- Office Of Air Force History 3
- Office Of little Dots 1
- Office Of The Chief Of Military History 1
- Office Of The Chief Of Military History United States Army 1
- Offices Of The Studio 1
- Oireacht Na Mumhan Books 3
- Oklahoma State Department Of Education 1
- Old Abbey Books 1
- Old Bakehouse Publications 9
- Old Comrades Association 1
- Old Forge Publishing 1
- Old Hall Press 1
- Oldbourne Book Co. Ltd. 9
- Oldbourne Press 1
- Oldhams Press Ltd 1
- Oliphant, Anderson & Ferrier 1
- Oliphants Ltd. 1
- Oliver & Boyd 8
- Oliver and Boyd 8
- Olivetti 1
- Omnibus Books 1
- On Parallel Lines Publishing 1
- Oneworld 2
- Orbis 2
- Orbis Publishing 2
- Orchard Books 17
- Oresko Books Ltd. 1
- Organisation For Economic Co-Operation 1
- Organisation For Economic Cooperation And Development 1
- Oriel Press 2
- Orient Longmans 1
- Oriental Press 1
- Orion 3
- Orion Books 2
- Orion Children's Books 10
- Ormeau Baths Gallery 1
- Orpheus Press 1
- Osprey Aerospace 6
- Osprey Automotive 10
- Osprey Aviation 9
- Osprey Military 2
- Osprey Publications Ltd 3
- Osprey Publishing 93
- Otava 4
- Other Criteria 1
- Otto Schulze And Company 1
- Otto Wigand 5
- Ouest France 1
- Overlook Duckworth 1
- Oxbow 1
- Oxbow Books 3
- Oxford 1
- Oxford at the Clarendon Press 7
- Oxford Historical Society 2
- Oxford Illustrated Press 1
- Oxford Publishing 30
- Oxford Publishing Co 24
- Oxford University Department For External Studies 1
- Oxford University Press 427
- Oxley-Sidey Publications 1
- Oxo Limited 2
- Oxonhoath Press 3