The Oxford comma (also known as the serial comma) is that extra comma that you sometimes get at the end of a list, before the and or the or. “She wrote novels, essays, and JavaScript” uses an Oxford comma. “He bought apples, butter and the ranch” doesn’t.
The Oxford moniker derives from the century-old endorsement of the serial comma by the Oxford University Press manual of style; and the OUP is backed up by a slew of revered authorities: Strunk’s Elements of Style, Fowler’s Dictionary of Modern English Usage and the Chicago Manual of Style. Why? Because omitting the Oxford comma can result in distressing double meanings:
“She lives with her two children, a cat and a dog.”
Legions of grammarians are quick to point out that while the lack of an Oxford comma can cause ambiguities, its presence never will. Here’s something we can all get behind, right?
Well, no, not really, because it turns out that for every phrase that the Oxford comma clarifies, there’s another for which it obfuscates. “Through the window she saw George, a policeman and several onlookers” clearly refers to two people and some onlookers. Throw in the Oxford comma and George has become a policeman: “Through the window she saw George, a policeman, and several onlookers”.