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