There has long been a raging civil war among grammarians and stylebook writers over it, and it’s in Seattle’s local news today (Friday, March 31, 2023) because the county council just voted unanimously to approve its use in official documents, or its stylebook, or whatever (see tweet with link to Seattle Times story here).
This is one of those arguments where you’re forced to choose sides — you either use the comma, or you don’t — and trust me, grammar fights can get ugly.
If you don’t know what an Oxford comma is, read a primer here. You’ve been using it (or not) all along; it’s not like an AK-47 (you know when it goes off), it’s stealthy, so you could smack somebody with one without knowing it, kinda like a bad parking job. And yes, I know that last sentence is bad grammar, but it doesn’t have an Oxford comma in it.