Publishers
- O.R. Reisland 1
- O'Reilly 1
- Oak Knoll Press 5
- 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. 3
- Odhams Books 2
- Odhams Press 2
- Odhams Press Limited 4
- Odhams Press Ltd. 5
- Odibourne Press 1
- Oesch Verlag 1
- Office For Official Publications Of The European Communities 1
- Office Of Air Force History 2
- 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 Bakehouse Publications 3
- Old Hall Press 1
- Old Pond Publishing 1
- Oldbourne Book Co. Ltd. 9
- Oldbourne Press 1
- Olfran Books 1
- Oliphant, Anderson & Ferrier 1
- Oliphants Ltd. 1
- Oliver & Boyd 8
- Oliver and Boyd 9
- Olivetti 1
- Omnibus Books 1
- On Parallel Lines Publishing 1
- Oneworld 2
- Orbis 2
- Orbis Publishing 2
- Orchard Books 15
- 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
- Original Writing Ltd 1
- Orion 3
- Orion Books 2
- Orion Children's Books 11
- Orkney Heritage Society 1
- Ormeau Baths Gallery 1
- Orpheus Press 1
- Osprey Aerospace 6
- Osprey Automotive 13
- Osprey Aviation 8
- Osprey Military 2
- Osprey Publications Ltd 2
- Osprey Publishing 91
- Otava 4
- Other Criteria 1
- Otto Schulze And Company 1
- Otto Wigand 5
- Ouest France 1
- Overlook Duckworth 1
- Oxbow Books 10
- Oxford 1
- Oxford Archaeology North 1
- Oxford at the Clarendon Press 6
- Oxford Historical Society 2
- Oxford Illustrated Press 3
- Oxford Publishing 32
- Oxford Publishing Co 41
- Oxford University Department For External Studies 1
- Oxford University Press 427
- Oxley-Sidey Publications 2
- Oxo Limited 2
- Oxonhoath Press 1
- Oyster Books 1