The Oxford comma, otherwise known as a serial comma, is an optional comma placed before the final “and” in a list. Its use is more stylistic than strictly necessary, but it can be helpful to clarify meanings.
Generally, I don’t tend to use Oxford commas. I suspect this is the kind of thing that is drilled into you at a young and impressionable age. If it hasn’t become a habit by the age of ten then it’s probably passed you by.
Most of the time Oxford commas are unnecessary. For example:
The colours of the Union Jack are red, white and blue.
There’s no need to write:
The colours of the Union Jack are red, white, and blue.
The Oxford comma is handy, however, in lists of compound items. For example:
The choice of sandwiches included tuna and cucumber, cheese and pickle, and bacon and tomato.
Thanks to the Oxford comma in the above sentence there can be no confusion about ordering the right sandwich.
It can also help avoid unintended meanings. Consider this sentence:
I love my children, Harry Potter and Hermione Granger.
The above sentence could be read as, my children are Harry Potter and Hermione Granger. An Oxford comma helps to make it clear that my children do not have any magical powers (alas):
I love my children, Harry Potter, and Hermione Granger.
Although for greatest clarity it would be best to write:
I love Harry Potter, Hermione Granger and my children.
But maybe one shouldn’t put one’s children after fictional characters 🙂