What is the Oxford comma, and do you need it?

The Oxford or serial comma is the one before the conjunction in a list of three or more items: eggs, milk, and bread. Newspapers following AP style generally omit it; book publishers and academic styles including Chicago and MLA use it. Both are legitimate conventions, which is why this argument never ends. The tiebreaker is ambiguity: without it, sentences occasionally collapse into unintended readings, as in the classic invitation thanking my parents, Ayn Rand and God, which the comma disambiguates instantly.

The argument has real money behind it. In 2017, a federal appeals court ruled in favor of dairy drivers in Maine in a dispute where the absence of a serial comma in an overtime exemption statute made the list genuinely ambiguous, and the company ultimately settled for millions. That case is now the standard citation for people who write contracts for a living. The practical position: use it consistently unless a style guide you are bound by forbids it, because including it costs nothing and prevents a category of error, while omitting it saves one character and occasionally changes the meaning of what you wrote. Consistency within a document matters more than which side you pick.