Grammar's great divide: When to use the Oxford comma

The 'Oxford comma' (also known as a serial comma or series comma) is an optional comma placed before the coordinating conjunction (usually and, or or nor) in a list of three or more items: We sell books, videos, and magazines. Should we use the Oxford comma? Can it really be optional, or is there a universal rule? TED-Ed explores both sides of this comma conundrum.