OnArt Bookstore  
   Books in many categories, with a wide selection in Art.
   http://www.onart.com/boo/bookstor.htm

Ox Bow Press  
   An independent, scholarly publisher dedicated to both new titles and reprints.
   http://oxbowpress.tripod.com/Index.htm

Opamp Technical Books  
   General book store; specialties include many technical topics.
   http://www.opamp.com/

OSS Publishing  
   Specializes in health, pet and family/education-related books.
   http://www.osspublishing.com

Otford Press  
   A publishing house based in Otford, NSW Australia.
   http://www.otfordpress.com.au

One Knowledge  
   Business, professional, technical, and educational publications.
   http://www.oneknowledge.com

Online Success Broker  
   selection of books on business, language and personal development.
   http://www.store.yahoo.com/onlinesuccessbroker

Open Door Books  
   Classic cookbooks, crafts, hobbies and music.
   http://www.opendoorbooks.com/

One World Books  
   Selection books and e-books; emphasis on southern African authors. On-line forum for smaller publishers and self-publishers.
   http://www.oneworldbooks.com/

OKDPM  
   Selection of books on recreation and hobby interests.
   http://www.okdpm.com/

Octopus Publishing Group  
   Style, food, drink, maps, cartography, children's, health, antiques, reference, and educational titles by UK publisher.
   http://www.octopus-publishing.co.uk/

Oxmoor House Book  
   Publishes craft, cooking, gardening, and decorating titles.
   http://oxmoorhouse.com

Open Court Books  
   Publisher of Eastern philosophy and thought along with western philosophy and popular culture.
   http://www.opencourtbooks.com