It goes beyond a simple comma but there is where we start. I do not like putting in the Oxford Comma. No! But when you have a subordinate clause before the comma it looks awfully like the Oxford Comma and then I start adding them to make them look uniform. You can use the Oxford Comma or not but, if you use it, use it all the time. First you have to learn to distinguish between a real Oxford Comma and a comma masquerading as an Oxford Comma.
“Kim was working, Mark was writing this article, and Alexis was on the phone.”
The comma before the “and” is an Oxford Comma. I could have easily written
“Kim was working, Mark was writing this article and Alexis was on the phone.”
It’s called an Oxford comma because the editors at Oxford University Press use it. The copycats at Harvard also use it. Some call it the Harvard comma but I wont name names.
I have three problems with not using the Oxford or Serial Comma. One of the main reasons I am converting to its usage is that Colette, my editor, uses it. That’s a big issue. The second one is the relative clause or subordinate clause before the conjunction.
“I am having a drink with Joan, who is married, and Big Terry.”
Granted that’s an awkward sentence but it’s the best one I can think of right now. That’s not really an Oxford Comma though it looks like one. We have found our masquerading comma. If I take out the relative clause “who is married” it would look like this.
“I am having a drink with Joan and big Terry.”
Two is not necessarily a big enough list to use the Oxford Comma. The Associated Press Stylebook recommends not using the Oxford Comma in a simple list.
“I had a drink with Joan, Terry and Donna.”
“I had a drink with Joan, Terry, and Donna.”
The third and final reason is putting in the Oxford Comma will help with ambiguity. Leaving it out wont.