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/

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

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/

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

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

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

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