O and I Books: Second-hand bookstore. Specializing in English and Spanish rare, hard to find antique 1876-1997 books.
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.
Oasis Used and Antiquarian Book Search: Buys and sells all type of fine used books, scholarly, history, technical, art, science, and fiction.
Observer's Swap Shop: Swap site for Observer's Pocket Series books published by Frederik Warne from 1937 to 1982. Membership required for full service.
Observermart: Specialises in Observer's Pocket Books published in the UK by Frederick Warne.
Old Book Company: Books, in all subjects, available through a search engine.
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.
OldBookShop.Com: Used, out of print, and rare hardback books.
Olde South Books: Used and rare books, including medical, genealogy, fiction, biography, and religion.
Olof Edlund: Antiquarian and rare books and manuscripts in all fields.
On the Shelf: Buys, sells and trade used books in all genres. Includes comic books, collectibles, series and first editions.
Owl at the Bridge: Scholarly, rare and unusual out-of-print books. Primary interests are Italian history and culture and American industrial silver.