I know that my US-based MacBook has NOAD as its dictionary reference. Do other parts of the world have the option of other built-in dictionaries? If so, is NOAD still the default? Does Great Britain have the option of the OED instead of NOAD, for instance?
All parts of the world have options of other dictionaries. To add (or remove) alternate dictionaries, launch the built-in dictionary application and open the app's preferences. From there you can add or subtract as many dictionaries as you'd like - including the Oxford English Dictionary.