(Also known as the serial comma.)
Avoid using an oxford comma except where it helps to clarify meaning.
The oxford comma can change the meaning of a sentence considerably. The oxford comma can also impact readability by introducing or resolving ambiguity. Because of this it is hard to give a fixed rule on their usage. The best rule to follow is probably:
If an oxford comma can alter the meaning of your sentence, you should probably rewrite your sentence.
If you would like to see internet strangers debating the use of the oxford comma then this post on Stack Exchange is a good place to start.