I sometimes have to adapt a text I have translated into BE to bring it into line with American punctuation rules. So if I have list of more than three things, I dutifully put a comma before and (as in: Spain, Portugal, and Germany are all EU member states). This is not the norm in the UK, of course, although some publications adopt it (especially the Oxford University Press). Sometimes, even in BE, it is helpful to add a comma before and. My question is about what percentage of Americans follow this "rule", even in the example I gave (where the use of the comma seems absolutely pointless to me). Are there Americans who never use the Oxford comma? Since it is only a convention (and not compulsory), I would be surprised if teachers corrected students' texts which did not follow the "rule". And on what grounds?