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.
You put a comma between the first and second objects in the list, it makes sense to put it between the last and second to last. That might just be my slight OCD talking though.
You put the word "and" between the last and second to last objects in the list, it makes sense to put it between the first and second.
"lions and tigers and bears"
So, this is the argument I heard for not using the oxford comma. The comma replaces each instance of "and" in the list. You don't replace the last "and", so no comma is needed.
"lions, tigers and bears"
However, I am definitely an oxford comma user.
The ambiguities that are created by its use are almost exclusively because of parentheticals which can be very easily re-punctuated or moved around in a sentence.
"I have visited Ohio (my home state), California, and Massachusetts in the United States; Quebec, Newfoundland and Labrador, and Ontario in Canada; and Trinidad and Tobago, Turks and Caicos Islands, and Antigua and Barbuda in the Caribbean."
Edit: Oxford comma or serial comma?