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: