What is an Oxford comma, you ask? When one lists three items and separates each with a comma, the final comma is known as an Oxford comma. Therefore, in the sentence "I will be a grammarian today, tomorrow, and forever after," the comma inserted before and forever after is an Oxford comma.
It is said in some circles that Oxford commas are unnecessary. I submit that it provides clarity to lists and can reduce embarrassing confusion. For instance:
And thus endeth the lesson. Amen.