Here’s my reasoning for pushing my clients to ALWAYS using the Oxford comma: If we get into the habit of always using it, at least we’ll be clear in the one or two sentences that might really need it, and we won’t have to think about it.
I have better things to do than to critique every sentence containing items in a series and try to decide if my reader(s) will understand my meaning. Always using the comma helps me be sure that if it needs to be there, it is. And if it doesn’t need to be there — well, it’s there anyway. Hardly anyone will notice.
And if leaving it out confuses a reader, we can’t then say “Well, you should have known what I meant!”
No. We really can’t.