And to prevent this from devolving into a bunch of stupid images pointing out funny situations that result from using or not using it, I will include them all: http://goo.gl/SUuFx
I was under the impression that the Oxford comma was part of standard usage in the USA. Is this not the case?
I only use it when things would otherwise get confused, e.g.: "The foo is available in red, white, and red and black."
Why? It's what I was taught at school (in the UK) and am therefore familiar with. I think some UK school districts follow Oxford, and other's don't, but I'm not sure what the split is.
I did spend a year at an American school as a kid, and I remember being confused about the extra comma. But nowadays I'm used to it.
I always use it because I like it, even though I was taught against it in school. Those times when I had to write something of consequence (books and scientific articles), I had editors to change it if that contradicted their MOS.