What is the Oxford comma, anyway?

The Oxford comma is a hotly-debated, free and easy  punctuation mark - as explained in this video from Ted Ed.
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Its use is basically optional; you can add it or not, as you see fit, as long as you are consistent within a single text.

And here's an amusing example of just how strongly some of us feel about it.

I have always had a lot of respect for this punctuation mark and its continued existence, against all the odds. 
Attention to this sort of detail is something you might want to look for in a translator.