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/

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

Old Book Company  
   Books, in all subjects, available through a search engine.
   http://www.oldbook.co.uk/

Old Catalogs and Books  
   Catalogs from the 1800s to 1970s, out of print engineering and medical books, machinery manuals, and company literature.
   http://www.oldcatalog.com/

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/

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/

Old Paper World  
   Rare books, magazines, newspapers, ephemera; a broad cross section of rarities in many fields.
   http://www.conknet.com/~oldpaperwrld/

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

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.
   http://www.observers-books.co.uk/

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/

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/

Over-Sands Books.  
   Varied stock of antiquarian, second-hand, and out-of-print books for sale. The Shop is situated at Grange-over-Sands, UK.
   http://OverSandsBooks.biblion.com/