An oxford comma is the comma right before the and in a series.
I have 100 pounds of iron, 50 pounds of steel, and coal. I have 100 pounds of iron, 50 pounds of steel and coal.
In the first example, the amound of coal is not specified, while in the second example there are clearly 50 pounds of coal. By omitting the oxford comma, you can let people think that you have 50 pounds of coal, even if you do not, as the oxford comma is often viewed as optional.
But why would you lie about how much coal you have? why would you lie about something dumb like that?