The New Oxford American Dictionary is not the same thing as the Oxford English Dictionary. The former is, don't get me wrong, a fine dictionary for 99% of what most people use a dictionary for. The latter is uncontested as the authoritative dictionary of the English language, and for folks who are into words it's fun to browse for the etymologies alone.
A printed set of the OED (20 volumes, 22,000 pages) costs about a grand. The CD-ROM costs $295, and subscribing to the online version costs $295 a year. It's not for everyone.
Thanks I understand now. It sounds like a serious hunk of book and perfect for the philology inclined.