The Oxford comma is an (optional!) comma which can be placed before placed before the "and" or "or" in a series of nouns. It is fiercely defended by (some) grammarians on the grounds that it can change the meaning of a sentence.
Why use an Oxford comma?
It supporters might point to the semantic difference between these two sentences:
a) The greatest influences in my life are my sisters, Oprah Winfrey and Madonna.
b) The greatest influences in my life are my sisters, Oprah Winfrey, and Madonna.
Taken literally, the first sentence could mean that Oprah and Madonna are sisters. This sensational interpretation is prevented by the Oxford comma in the second sentence.
So Oxford comma enthusiasts are correct?
An alternative view would be that such examples of potential confusion are vanishingly rare. Opponents of the Oxford comma consider it to be fussy, unsightly and/or pedantic.
So why not just make it optional?
The English language does not have an academy to control or police 'correct' usage. Some structural features underpin how it functions - a first language speaker would not say or write I saw the cup blue intuitively grasping that an adjective normally precedes a noun.
Other conventions, like the Oxford comma, are essentially a matter of personal preference. Journalists should be advised, however, that 'optional' is not a word that appeals to stylebook editors.