The Oxford comma–sometimes referred to as the Havard comma– commonly causes confusion in written English. Its use is not obligatory, with some style guides asking for it and others not. As a general rule, it tends to be in more frequent use in American written English than in the UK, Australia, or Canada. It is also important to remember that news outlets that use AP style will omit its use.
The most important thing with written style is consistency, so if in doubt, it’s better either to use it always, or exclude its use completely. The reason it’s a useful style element is that it can clear up confusion in sentences.
In the example: “We invited the rhinoceri, Washington and Lincon”
– It could be interpreted that Washington and Lincon are rhinoceri! Hence we need to use the Oxford comma in order to be clear.
“I’d like to thank my parents, Ayn Rand and God”
– Again, it is not immediately obvious in written English that the parents’ names are not Ayn Rand and God!
Check out the info graphic below on the Oxford Comma from onlineschools.com for more information: