As a general rule, do not use the serial/Oxford comma: so write 'a, b and c' not 'a, b, and c'. But when a comma would assist in the meaning of the sentence or helps to resolve ambiguity, it can be used -- especially where one of the items in the list is already joined by 'and'.
Update: This document is from a branding style guide for Oxford University. It recommends against using the Oxford comma in most cases. The Oxford Style Manual, meant for the general public and last published in 2003 by Oxford University Press, "a department of the University of Oxford", recommends using the Oxford comma in all cases. So basically, Oxford is telling us to use the Oxford comma but isn't going to use it internally. Does this all make sense?
The New Oxford Guide to Writing