It is now mandated that one use the Oxford Comma, properly known; it is commonly known as the serial comma. There are those of us who remember the days when use of the serial comma was optional. Options, at least in public discourse, are increasingly rare.
The age-old saw, which never did cut right, advises that you cannot have both security and liberty, that the two together make one pie, i.e., you can have lots of liberty, but not much security; you can have lots of security, but not much liberty. Society argues about how much of each we shall have, and, depending on the size of the pie, we come out better or worse, or both.
The reason we must use the serial comma is to guarantee security, at least in written expression. In the second place, now that it is mandated, if you do not use it, grammar police will have your hide. In the first place, your readers cannot be trusted to read without suspicion. They have been trained to operate quite criticizingly, an attitude which is similar to critically, but is morally distinct and disgusting.
It used to be that a reader was trained to read empathetically, no matter the author or the subject, so if he or she was wont to use the serial comma, that was okay, and if he chose not to use the serial comma, instead allowing for ambiguity in writing, she was entrusting the reader the task of interpretation in favor of clear communication.
No, we have become lazy, solipsistic moralists, with the goal of browbeating each other into conformity, or at least conversating to exhaustion into compliance, all character expunged, all eccentricities marginalized, merciless readers. This is what comes of forcing the political, which needs be merciless, into almost every aspect of daily life, where mercy is much needed. Where isn’t political language? In sex and grocery shopping it is; therefore, we must use the Oxford Comma. Thou shalt.
I’m referring, of course, to this, this, and this, among other things.