Also known as the serial comma, the Oxford comma is used before and and sometimes before or in a list of three or more items in a sentence. It makes sentences less confusing when you incorporate the Oxford comma.
In Karen's acceptance speech, she thanked her parents, Justin Timberlake and Dr. Ruth.
Writing it this way — without a comma between Justin Timberlake and Dr. Ruth — can be misinterpreted to sound like JT and Dr. Ruth are Karen's parents.
Here's what that sentence should look like
for clarity's sake:
In Karen's acceptance speech, she thanked her parents, Justin Timberlake, and Dr. Ruth.
A comma before Dr. Ruth lets us know that Karen thanked four people instead of two. Big difference, right?
You can also rearrange the people listed in the sentence:
In Karen's acceptance speech, she thanked Justin Timberlake, Dr. Ruth and her parents.
When you reword the sentence in this way, you can omit the serial comma without confusing readers.
Here are some other examples:
He ordered five burgers, two sides of fries, three cokes and two large milkshakes.
In this case, you can leave that last comma out and the sentence wouldn't be weird because the use of the Oxford comma is a matter of style (house style, CMOS, AP, etc.).
The way I see it, though, you can never go wrong using the serial (Oxford) comma because if you get accustomed to not using it, you may need it one day and forget it, making your sentence look like the first example I gave you or like this one:
Doug's wife sent him to the store to pick up bananas, bread, milk, cheese and cereal or oatmeal.
Is Doug supposed to pick up every item on his list . . . or not? The words and and or are seen near the end of this sentence with no commas in sight. We cannot be certain of the exact items Doug's wife wants from the store, so let's help him out by throwing a comma in there:
Doug's wife sent him to the store to pick up bananas, bread, milk, cheese, and cereal or oatmeal.
With the comma added before and we now know that Doug's wife wants all of the items listed in this sentence, and that he can decide between cereal and oatmeal while he's at the store.
If you need more help understanding the Oxford comma, feel free to message me in the chat bubble located in the bottom right-hand corner of this screen, or you can email me at firstname.lastname@example.org.
I will likely post a more in-depth blog post about Oxford commas at a later date, but until then, please remember that writing is all about consistency. If you choose to use the Oxford comma in your writing, make sure that you use it consistently throughout your piece.