Comment by Stratoscope

Most of the time, you can use the Oxford comma, the non-Oxford comma or any other kind of comma, and it just doesn't make any difference.

But sometimes you need it. Here's an example:

"There are two hard problems in Computer Science: naming things, cache expiration, and off-by-one errors."

Take out the comma and you ruin the joke - at least to my ears:

"There are two hard problems in Computer Science: naming things, cache expiration and off-by-one errors."