The first thing you need to know about the Oxford Comma is actually a thing you need to know about Wikipedia. Wikipedia is a special place on the internet where the conversation the rest of us are having about many topics can’t actually happen. In this case, Wikipedia has determined that the Oxford Comma is actually called the Serial Comma, even though nobody in the whole world calls it this. Somewhere in the history of the current Wikipedia article on this topic, somebody discovered primacy of the word “serial” and insisted that since that word was used first, in 1121 or something, it beats “Oxford.” This silly sort of thing is common in Wikipedia, and I’ve discussed it before. But, I digress.
The Oxford Comma, also known as the Harvard Comma, is that last comma in a list before the “and” or the “or.”
You need it in sentences like this:
“I took a picture of my parents the president and the first lady.”
There are different ways to put commas in that sentence. In one version of this sentence, I am Chelsea Clinton. In the other version of this sentence, I’m a visitor to the White House and the President and First Lady are standing there next to my parents, and I snap a photograph.
Most style guides for writers, including the Oxford Style Manual, the APA, Chicago Manual of Style, MLA, Strunk and White, the USGPO and others mandate the use of this ultimate comma. It is less often used in the UK, but the Oxford guide does require it. Overall, the total number of words printed under stylistic guidance use it, though the Oxford comma is not accepted stock and barrel.
The AP style guide eschews the Oxford comma, and generally, journalistic style guides recommend against it.
The reason a style guide would require it is simple, in my view. A missing Oxford comma is an abomination. You can really screw up the meaning of a sentence by leaving it off. But it is hard to mess up a sentence by using it when it is not needed. Most of the time, the only reason to not use it is because you are short of commas. In the old days, when commas were an actual thing, made out of lead, maybe it was worth having a rule that helped conserve them. Perhaps that is also why American spelling tends to diverge from British spelling in the use of fewer letters per word. They had lots of lead type in Great Britain because of a longer history of of type use, or because they shifted to fighting all their wars overseas.
The point is, if you tell your writers to always use it, mistakes will rarely be made.
There are, of course, times when the Oxford Comma adds, rather than subtracts, ambiguity from a sentence.
“I am going to say a prayer for my dog, Jesus, and my cat.”
Am I praying for three entities (two of which exist), or am I praying for my dog, named Jesus, and also, my unnamed cat?
Which brings us to my own personal conclusion about the Oxford-Harvard comma. Which, by the way, is not too different from what each and every style guide actually says.
Most style guides say, “Use/Don’t use the Oxford Comma, unless you must, to make your meaning clear.”
My preference is to say, “Use/Don’t use the Oxford Comma as you please, just make sure your meaning is clear, unless, as a writer, you chose to be unclear.”
This sort of approach is true with punctuation generally. You really should start your sentences with Upper Case letters, and end them with periods. But, you don’t have to. You can be e E cummingS. How does a writer handle quotes within quotes? Up to you. According to many style guides, when you shift paragraphs within dialog, normal quotation rules fly out the window. But, you can do it differently if you want.
Some writers use a lot of semicolons; others do not. There is no rule that forces you to do so, but there are guidelines for when you could. Which you can ignore; I don’t.
Is your text meant to be read aloud, even if just in the head of the reader? Is there cadence that affects the way the words feel? If so, you might find yourself using, as needed, commas that technically should not be there. Or you might leave some off to change the sound your words make in someone’s head. That is your choice as a writer. Seriously. That is your choice, as a writer.
Non fiction that is actually published almost always has to pass through the style guide process, so writers are advised to know the guide they are working under, and adapt accordingly, even if it hurts. Fiction, even if processed by a publishing system, is more personalized. You should be able to argue for your own style because that is the whole point of getting a writer to write something instead of just doing it yourself. Right?
The truth about the Oxford, Harvard, or Serial comma is that there is always optional but usually recommended. Saying you can’t use it unless necessary will cause people to err on the side of caution with their editor, but against caution with their readers. Saves you commas, but in the long run, causes you more trouble than it is worth.