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’:
They had a choice between croissants, bacon and eggs, and muesli.There are some cases where the comma is clearly obligatory:
The bishops of Canterbury, Oxford, Bath and Wells, and Salisbury.
But still the controversy rages.