The serial comma (also known as the Oxford comma or Harvard comma) is the comma used immediately before a coordinating conjunction (usually ‘and’ or ‘or’) preceding the final item in a list of three or more items.
For example, a list of three countries can be punctuated as either ‘Portugal, Spain, and France’ (with the serial comma) or as ‘Portugal, Spain and France’ (without the serial comma).
Opinions vary among writers and editors on the usage or avoidance of the serial comma. In American English, the serial comma is standard usage in non-journalistic writing that follows the Chicago Manual of Style. Journalists, however, usually follow the AP Stylebook, which advises against it. It is used less often in British English, where it is standard usage to leave it out, with some notable exceptions such as ‘Fowler’s Modern English Usage.’ In many languages (e.g. Danish, Dutch, French, German, Greek, Italian, Polish, Romanian, Spanish) the serial comma is not the norm and may even go against punctuation rules. It may be recommended in many cases, however, to avoid ambiguity or to aid prosody (rhythm of speech).
In addition to resolving ambiguity, the oxford comma matches the spoken cadence of sentences better than the alternative. However, it is redundant in a simple list because the ‘and’ or the ‘or’ is often meant to serve (by itself) to mark the logical separation between the final two items unless the final two items are not truly separate items but are two parts of a compound single item. Many sources are against both systematic use and systematic avoidance of the serial comma, making recommendations in a more nuanced way.
The Daily Omnivore 
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