The thing I hate about Oxford comma apologists is that they always imply or explicitly say that the Oxford comma removes ambiguity (and it often comes across a bit obnoxious or pretentious). It's happened in this very thread. But the thing is, it's untrue. There are situations where the Oxford comma helps, but there are also plenty of situations where using it creates ambiguity. Formal writing should always seek to minimize ambiguity and therefore it should use or omit the Oxford comma as necessary.
In the case where the Oxford comma makes no different, I usually prefer to leave it out. If it doesn't remove ambiguity, it's just useless ornamentation.
Also, there are some people who take it too far and use the Oxford comma in lists of two. Gross.
We should have simply designed a language where all lists are comma-delimited and their and/or aspect is determined at the start of the list, like:
\/(a_i, i=1..n) [i.e., OR a_1, a_2, ..., a_n]
/\(a_i, i=1..n) [i.e., AND a_1, a_2, ..., a_n]
no need for confusion or ambiguities.