Since the value in adding it only to the options bag is quite small (for it would require an app developer to develop a UI for the user to control their oxford comma preference and then pass it to the constructor of ListFormat all around their app), I believe that this feature should be primarily handled by the language tag in CLDR first, and we can then consider adding support for it into ECMA402.
It is always valuable to give the option to developers to use it how they want. The developer may want to use it without necessarily making it a user-controllable UI preference!!
