The Oxford comma, also known as the serial comma, is the (optional) final comma that comes before the last item in a series. Because it is optional, its rules are often confusing for English language learners. Oftentimes, those who swear by the Oxford comma do is because their grammar or writing teachers insisted on using it!
Although it is optional, the Oxford comma does serve a very important purpose: identifying whether the last two items are two separate items within the list, or whether they are identifying a previous item with an appositive (a noun, noun phrase, or series of nouns next to a word to identify or rename it).
See the problem?
When speaking, people cannot see the punctuation being used. They will have to use the rest of the conversation to determine whether there are four party attendees or whether they are two! They might have to ask additional questions in order to find out. In writing, the optional Oxford comma clarifies the entire issue with the smallest of penstrokes.
So what’s the final word? It’s always better to use the Oxford comma, just in case.