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?
MacOS – Do all editions of OS X use the New Oxford American Dictionary
dictionarymacos
Related Solutions
Many people have tried to find the files for Apple's spell check dictionaries but I don't recall any successes.
As for the Language tab in Language & Text, it really doesn't have anything to do with spelling as such, although the top item on the list can set the default spell check dictionary in some circumstances. The top item on the list also sets the OS and app language (for the 22 that are available), the default collation order, and the preferred language requested by Safari. The order of the list can determine the app language used if a localization for the top item is not available. The order can also determine whether a Japanese or Chinese font is used for text which is ambiguous in that regard. I think adding an item to the list can sometimes provide additional encodings in Mail app.
The dictionaries are stored in...
/Library/Dictionaries
~/Library/Dictionaries
Best Answer
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.