Publishers
- O.D.E.J. 1
- O.P.C. 1
- O.R. Reisland 2
- Oak Knoll Press 5
- Oakmagic Publications 2
- Obelisk Publications 1
- Oberon Books 1
- Oblong 2
- Obv 1
- Ocean Boos Pvt. Ltd 1
- Octopus 3
- Octopus Books Ltd. 4
- Odhams Books 2
- Odhams Press 3
- Odhams Press Limited 5
- Odhams Press Ltd. 6
- Odyssey Books 1
- Oesch Verlag 1
- Office Du Livre 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
- Ohio State University Press 1
- Old Bakehouse Publications 13
- Old Hall Press 1
- Old Motor Magazine 1
- Oldbourne Book Co. Ltd. 17
- Oldbourne Press 1
- Oliphant, Anderson & Ferrier 3
- Oliphants Ltd. 4
- Oliver & Boyd 14
- Oliver and Boyd 12
- Olivetti 2
- Omega Books 2
- Omnibus Books 1
- ONE 1
- Oneworld 1
- Ophrys 1
- Orakau House 1
- Orbis 3
- Orbis Publishing 2
- Orbit 1
- Orchard Books 16
- Oresko Books Ltd. 1
- Organisation For Economic Co-Operation 1
- Organisation For Economic Cooperation And Development 1
- Oriel Press 1
- Orient Longmans 2
- Orion 6
- Orion Books 2
- Orion Children's Books 16
- Orion Media 1
- Orpen Press 1
- Orpheus Press 1
- Osboldstone & Co. Pty. Ltd 1
- Oscar Blackford 1
- Osprey Aerospace 10
- Osprey Air Combat 2
- Osprey Automotive 8
- Osprey Aviation 7
- Osprey Military 3
- Osprey Publications Ltd 4
- Osprey Publishing 71
- Other Criteria 1
- Ottenheimer Publishers, Inc. 1
- Otto Maier 1
- Otto Schulze And Company 2
- Otto Wigand 5
- Our Dogs Publishing Co. Ltd. 1
- Outskirts Press 1
- Overlook Duckworth 2
- Oxbow Books 1
- Oxford 1
- Oxford at the Clarendon Press 17
- Oxford Clarendon Press 2
- Oxford Historical Society 2
- Oxford Illustrated Press 2
- Oxford Publishing 68
- Oxford Publishing Co 14
- Oxford University Department For External Studies 1
- Oxford University Press 449
- Oxo Limited 1