MacOS – Where is the New Oxford American Dictionary stored

It looks like this is possible in iOS 7. When you click to define a word, in the bottom left there is a "Manage" button. Here, you can tap the X for a dictionary that you want to delete from your device and tap the cloud button to download a dictionary you want. So in your case, tap the X to delete the NOAD from your iPhone, and tap the cloud to download the OED to your iPhone.

Do other parts of the world have the option of other built-in dictionaries? 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.