Open Court Books - Publisher of Eastern philosophy and thought along with western philosophy and popular culture. - http://www.opencourtbooks.com
Oxmoor House Book - Publishes craft, cooking, gardening, and decorating titles. - http://oxmoorhouse.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/
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/
OSS Publishing - Specializes in health, pet and family/education-related books. - http://www.osspublishing.com
Opamp Technical Books - General book store; specialties include many technical topics. - http://www.opamp.com/
OnArt Bookstore - Books in many categories, with a wide selection in Art. - http://www.onart.com/boo/bookstor.htm