A question was recently raised on OEDILF about the use of the Oxford comma (a comma before a conjunction) and I posted a reply which I will share with you. The query in fact related to the use of the comma before "or" and, as I was writing my reply, I suddenly wondered whether the rule is the same as it is before "and".

This is what I said and I would welcome others' thoughts.

"...The use of a comma before a conjunction (usually "and" but occasionally "or") is one that is frequently discussed on language boards. Most of us were taught at school that the "rule" is "...never use a comma before and..." and it is, like so many such rules, a good working hypothesis.

However, there are times when the use of such a comma (known as an Oxford comma in the UK and Harvard comma in the USA) is essential, as in the famous wedding speach:

"I would like to give thanks to my parents, Fred and Mary, and God". The omission of the final comma creates an ambiguity which is, to say the least, questionable!..."

It would maybe be worthwhile discussing this on Wordcraft.

RE's example (emph. added): "I would like to give thanks to my parents, Fred and Mary, and God".

The example might make one think the source of the ambiguity lies in the doubling of 'and'. However, I'd think it can arise, whether or not the conjunction repeats, in any listing where the next-to-last item is multi-word. For example, the following sentence would be less clear without the emphasized commas, even though it uses 'and' only once:

"Enron collapsed due to failure of its president, its board, and its outside auditors."

In contrast, this rewrite would work well even without that final comma:

"Enron collapsed due to failure of its president, board_ and outside auditors."

To us Yanks, the Oxford comma is better known as the serial comma or the Harvard comma. I learned to use it early on, and continue to this day.

I must have learned to use it because I do use it, for exactly the reason discussed in World Wide Words (thanks, jheem!). However, I have noticed that my editors will often try to remove it, and they have always been American.

As Michael Quinion says, he would normally not use the Oxford/Harvard/serial comma. However, since he writes for the OUP he has to conform to their style guide.

Similarly, Kalleh would normally use the serial comma, but might be forced to remove it to conform to her publisher's style.