A comma worth five million dollars
Most punctuation arguments are matters of taste. This one has a price tag. In 2017 the United States Court of Appeals for the First Circuit decided O'Connor v. Oakhurst Dairy, a case in which a group of Maine delivery drivers claimed years of unpaid overtime. The entire dispute turned on a missing serial comma.
Maine's overtime law exempted workers involved in "the canning, processing, preserving, freezing, drying, marketing, storing, packing for shipment or distribution of" certain foods. Without a comma before "or distribution," the phrase can be read as packing for shipment or distribution — one activity — rather than as packing and distribution as two separate exempt activities. The drivers distributed; they did not pack. Judge David Barron sided with them in a decision that opens by saying, in plain words, that the case exists for want of a comma. The dairy settled for $5 million across 127 drivers.
Why style guides disagree on purpose
The split is not sloppiness — it is two different professions optimizing for two different failure modes. The Chicago Manual of Style, the Oxford University Press and most book publishing use the serial comma, because their nightmare is ambiguity in a sentence a reader cannot ask about. The Associated Press and most newspaper style drop it, because their nightmare is a column inch and a reader who stops reading.
Both are right about their own constraint. That is why no amount of arguing settles it: the question "should you use the Oxford comma" is incomplete until you say what you are writing and for whom.
The famous examples cut both ways
Advocates love the apocryphal dedication — "to my parents, Ayn Rand and God" — which the comma would rescue. Opponents answer with sentences the comma breaks: "to my mother, Ayn Rand, and God" now reads as though the mother is Ayn Rand. The honest conclusion is that a comma cannot fix a badly ordered list, and that either convention can be made to look ridiculous by someone choosing the example.
One warning about this match
"Oxford comma" is one of the most searched phrases in this category, and not all of that traffic belongs to the debate: a great many people are looking up what the thing is, and a fair number are looking for the 2008 Vampire Weekend song of the same name. So treat the popularity of the question as a measure of curiosity, not of controversy.
The controversy is real all the same, and it has an unusual quality: it is one of the few style disputes where both camps are professionally trained, fully aware of the other's reasoning, and still not moving. Vote for the one you actually use when nobody is checking.
Sources: O'Connor v. Oakhurst Dairy, No. 16-1901 (1st Cir. 2017), opinion by Judge David Barron; reported $5 million settlement covering 127 drivers; Chicago Manual of Style and AP Stylebook on the serial comma; Maine overtime exemption statute at issue in the case.