Opinions vary about its utility but I prefer to use Oxford comma whenever I can. It’s that optional comma before the word ‘and’ at the end of a list (also known as serial comma). For example, “Ina, Mina and Dika joined us.” can be written as “Ina, Mina, and Dika joined us.” with an additional comma a.k.a. the Oxford comma. In this particular example, it doesn’t add anything to clarify the meaning of the sentence, but it does improve prosody. In some other cases, an Oxford comma does help remove ambiguity. For example:
My sisters, Ina and Mina joined us.
This sentence is ambiguous because it’s unclear whether Ina and Mina are my sisters. If they are, then only two people joined us. Otherwise, two people in addition to my sisters (however many there may be) joined us. We can add Oxford comma before the word ‘and’ to remove this ambiguity:
My sisters, Ina, and Mina joined us.
Now it’s clear that Ina and Mina are two people – in addition to my sisters – who joined us. If you’re not convinced yet, see the comic below that highlights the usefulness of Oxford comma: