Publishers
- O.P.C. 2
- O.R. Reisland 1
- O'Reilly 1
- Oak Knoll Press 3
- Oaklands 1
- Oakmagic Publications 2
- Oakridge Historical Research Group 1
- Obelisk Publications 1
- Obv 1
- Ocean Boos Pvt. Ltd 1
- Octopus 3
- Octopus Books Ltd. 3
- Odhams Books 2
- Odhams Press 2
- Odhams Press Limited 5
- Odhams Press Ltd. 2
- Oesch Verlag 1
- Office Du Livre 1
- Office For Official Publications Of The European Communities 1
- Office Of Air Force History 2
- Office Of little Dots 1
- Office Of Public Information, Hq, Roka 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
- Oklahoma State Department Of Education 1
- Old Bakehouse Publications 28
- Old Comrades Association 2
- Old Comrades Of The 11th Hussars 1
- Old Forge Publishing 2
- Old Hall Press 1
- Old House Books 1
- Old Line Graphics 1
- Old Motor Magazine 1
- Oldbourne Book Co. Ltd. 10
- Oliphant, Anderson & Ferrier 3
- Oliphants Ltd. 1
- Oliver & Boyd 9
- Oliver and Boyd 8
- Omnibus Books 1
- On Parallel Lines Publishing 2
- ONE 1
- Oneworld 2
- Orbis 3
- Orbis Publishing 5
- Orbis Publishing Ltd. 1
- Orchard Books 15
- Ordnance Survey 1
- Oresko Books Ltd. 1
- Organisation For Economic Co-Operation 1
- Organisation For Economic Cooperation And Development 1
- Oriel Press 2
- Orient Longmans 1
- Orion 4
- Orion Books 2
- Orion Children's Books 13
- Orkney Heritage Society 1
- Orlando Hodgson 1
- Ormeau Baths Gallery 1
- Orpen Press 1
- Orpheus Press 1
- Oscar Blackford 1
- Osprey Aerospace 7
- Osprey Automotive 16
- Osprey Aviation 7
- Osprey Military 3
- Osprey Publications Ltd 4
- Osprey Publishing 114
- Otava 4
- Other Criteria 1
- Otto Maier 1
- Otto Schulze And Company 1
- Otto Wigand 5
- Ouest France 1
- Outer Station Project 1
- Overlook Duckworth 1
- Oxbow 1
- Oxbow Books 7
- Oxford 3
- Oxford at the Clarendon Press 10
- Oxford Historical Society 2
- Oxford Publishing 41
- Oxford Publishing Co 20
- Oxford University Department For External Studies 1
- Oxford University Press 388
- Oxley-Sidey Publications 1
- Oxo Limited 2
- Oxonhoath Press 1