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

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/

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

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/

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

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/

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

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/