What is an Oxford comma and how do you use it? [duplicate]

I didn't even know this existed. What is it? How do you use it?

An oxford comma is a comma that is placed after the penultimate item [second or last] in a list before an 'and' or an 'or'. This only applies to lists of three or more items. Oxford commas are completely optional and remain at a matter of dispute, some people use them, while others don't.

Example

• I bought cheese, crackers, and a sandwich.
• I like cheese and crackers is correct, the rule of the oxford comma only applies to sentences of that are three or more items long
• There were many animals at the Bronx Zoo, tigers, monkeys, toucans, and orangutans were my favourite. Notice the comma after 'toucans'.