The Oxford comma, sometimes also called the serial comma, is used to clear up confusion in sentences with lists. It’s often a matter of preference whether you use it, but it can help to ensure clarity. Take the following popular example:
We invited the clowns, JFK and Stalin.
Without an Oxford comma, this looks like you are saying that JFK and Stalin are clowns (or that you’ve invited clowns called JFK and Stalin)!
So, we would write:
We invited the clowns, JFK, and Stalin.
This makes it clear that JFK and Stalin have been invited as well as the clowns.
Not Always Necessary
It really depends on what you are writing though. Take this sentence:
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We bought apples, mangos and bananas.
There’s really no danger of confusion in this sentence and so, if you like, you can leave out the comma before the final ‘and’ here. In British English, the Oxford comma is typically only used to prevent confusion. But in American English, it is fairly common to use an Oxford comma in any list.
Revising Your Sentence
You can also get around having to use the Oxford comma by simply restructuring your sentence. Our first sentence would be much clearer like this:
We invited JFK, Stalin and the clowns.
Conventions
Generally, British and Australian writers are less likely to use the Oxford comma and certain American newspaper outlets also leave it out. Some style guides may suggest its use though, so check yours to make sure. The best thing to do is to ask yourself ‘is my sentence clear as it is?’ If not, you might want to stick a comma in it!