These three sentences sum up why you should always use the Oxford comma

An Oxford, or serial, comma is the last comma in a list, which goes before the word "and." It's, technically, grammatically optional in American English.

However, depending on the list you are writing out, omitting it can lead to some confusion. A Reddit user found what could be the best three sentence argument in existence for the use of the Oxford comma:

reddit / user limabeanns

It would be cool if Ayn Rand and God were your parents, though.

It's likely the book is dedicated to the author's parents, Ayn Rand, and also God - but then again, it could be that the author's parents are named Ayn Rand and God.