Oxford comma

英語で3つ以上の事項をコンマで列挙するとき、最後の事項の直前に入れる and の直前にはコンマを入れるスタイルとコンマを入れないスタイルがある。それぞれの長所・短所と、どのスタイル・ガイドがどちらのスタイルを勧めているかについては Serial comma - Wikipedia が参考になる。

Oxford University Press はコンマを入れるスタイルを使っているらしく、Oxford comma という言葉さえある。
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The 'Oxford comma' is an optional comma before the word 'and' at the end of a list:

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It is so called because it was traditionally used by printer's readers and editors at Oxford University Press. Sometimes it can be necessary for clarity when the items in the list are not single words:

These items are available in black and white, red and yellow, and blue and green.

Some people do not realize that the Oxford comma is acceptable, possibly because they were brought up with the supposed rule (which Fowler would call a 'superstition') about putting punctuation marks before and.

ところが、肝心の Oxford University Press から出ている Peter Swan の Practical English Usage は、列挙する事項がそれぞれ短い場合（1語や2語の場合か）には通常コンマは使わないとあっさり説明している。

We can use commas to separate items in a series or list. A comma is not usually used with and between the last two items unless these are long. Compare:

I went to Spain, Italy, Switzerland, Austria and Germany.

You had a holiday at Christmas, at New Year and at Easter.

I spent yesterday playig cricket, listening to jazz records, and talking about the meaning of life.

(p.477)

Practical English Usage (Practical English Usage, Third Edition)

いや、別にオチはありません。