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Oak Knoll Books - Specialists in Books About Books; out-of-print books about book collecting, book history and the book arts; publisher, distributor, and bookseller.
http://www.oakknoll.com/

Oasis Used and Antiquarian Book Search - Buys and sells all type of fine used books, scholarly, history, technical, art, science, and fiction.
http://www.bookreader.org/

Olde South Books - Used and rare books, including medical, genealogy, fiction, biography, and religion.
http://www.oldesouth.net/

Olof Edlund - Antiquarian and rare books and manuscripts in all fields.
http://www.edlund-books.com/

O and I Books - Second-hand bookstore. Specializing in English and Spanish rare, hard to find antique 1876-1997 books.
http://www.oibooks.com/home.html

Old Books - A general antiquarian shop with as first printings of literature and children's books, 19th and early 20th century scientific and other periodicals.
http://www.oldbooks.net/

Owl at the Bridge - Scholarly, rare and unusual out-of-print books. Primary interests are Italian history and culture and American industrial silver.
http://www.owlbridge.com/

OldBookShop.Com - Used, out of print, and rare hardback books.
http://www.oldbookshop.com/

Observermart - Specialises in Observer's Pocket Books published in the UK by Frederick Warne.
http://www.observermart.co.uk/

On the Shelf - Buys, sells and trade used books in all genres. Includes comic books, collectibles, series and first editions.
http://www.ontheshelfbooks.com/