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’.
YES. I AM SO EXCITED ABOUT THIS.
RIP Oxford Comma
Oxford Writing and Style Guide no longer recommending the Oxford comma: