The Oxford comma (or serial comma)
Also known as the Harvard comma, the Oxford comma is the comma used between the penultimate item in a list and the and or or that connects it to the final item.
Its usage generally differs depending on which side of the Atlantic the author is sitting. It is normally included in American English texts and is often omitted, quite correctly, in British English writing, especially in newspapers and the like, which often have quite distinctive stylistic guidelines.
However, as the name suggests, use of the Oxford comma is also expected according to many British style guides, such as Oxford University Press. In short, if you aren’t sure whether to use it or not, the safest bet is to do so.
Example of the use of the Oxford comma
I have packed a basket with sandwiches, cakes, bottles of beer, crisps, and peanuts for our picnic today.
as opposed to
I have packed a basket with sandwiches, cakes, bottle of beer, crisps and peanuts for our picnic today.
Both are considered correct in British English.
Example of how the Oxford comma can clarify a text
However, in the interest of clarity, the Oxford comma can be included in lists of items that include and or are more complicated.
My favourite crisp flavours are roast chicken, salt and vinegar, and cheese and onion.
as opposed to
My favourite crisp flavours are roast chicken, salt and vinegar and cheese and onion.
More complicated sentences also require the Oxford comma to try and keep their meaning clear.
My favourite flavour is pickled onion, hers is roast chicken, and her Dad loves prawn cocktail.