I’m not sure if it’s actually referred to as “the Oxford comma,” but I heard it on Stephen Colbert last night, so I’m running with it. The point is, it’s quite often the subject of debate. What is the Oxford comma? It’s that last comma separating the items in a series. For example, in a, b, and c, the Oxford Comma would be the one following “b.”
According to current grammar rules, the Oxford comma should always be there.
This is confusing to some people, largely because it used to be optional. However, it can completely change the meaning of a sentence sometimes if it’s missing, and it’s never more obfuscating if it’s there, so why not always put it there?
Entire lawsuits have been built around a missing Oxford comma. Consider an estate that’s to be equally split among Jane, Jack, Millie and Joe. Does that mean Jane and Jack each get 1/3 of the estate while Millie and Joe split the remaining 3rd or that each of them gets a quarter of the estate? You may laugh, but it’s true.
The point is, especially in formal writing, it’s better to take the safe route. MLA and APA guidelines call for every item in a series to be followed by a comma, so no matter who you’re writing for, you’re better off assuming it’s not optional.