My first editorial job, where I worked 1977-1981, insisted on the Oxford comma. Subsequent jobs didn't, following the AP styleguide, which I recall saying "no comma before 'and'."

So having been constrained both ways, seeing the negative effects of "no exceptions!" policy, I decide case by case, thwi.

If the choice isn't immediately obvious, I read the sentence out loud. That generally solves the matter, at least for me.

In other words, whatever benefits the reader.

I'm more annoyed by misused quotes, dashes and capitals. In the previous sentence, I originally had a phrase about a capitalization practice, instead of just the word "capitals." I feel the Oxford comma would have been called for then.

In extreme cases for clarity, use a bullet list :)