Few topics on punctuation provoke stronger opinions or more angst than the Oxford comma. Those who advocate for its demise square off passionately against fans equally determined to save it.
This comma received its name from Oxford University Press, where printers and editors used it in publications. Today, many style guides call for the Oxford comma, including The Chicago Manual of Style, Modern Language Association Style Manual, American Medical Association Manual of Style, U.S. Government Printing Office Style Manual, Publication Manual of the American Psychological Association, and the venerable Elements of Style.
However, The Associated Press Stylebook, The New York Times, and The Economist omit the Oxford comma, claiming it's unnecessary and wastes space. Ironically, while Oxford University Press continues to use the comma, Oxford's public relations department does not. Neither does the rest of the United Kingdom, Canada, and Australia.
Whether I include the Oxford comma depends on my clients. If they follow The Associated Press Stylebook, then I omit it. If left to my own devices though, I always use it for two reasons.
It clarifies meaning. While the extra comma is not strictly necessary for understanding simple lists such as "she wore a leotard, tights, and a tutu," the meaning may be ambiguous in more complex lists. For example, "I want to thank my teachers, Eliza and Lauren." Are the teachers Eliza and Lauren? Or is the speaker thanking the teachers, Eliza, and Lauren?
Imagine a will in which the estate is divided among "James, Annie and Joy." Does James get one half of the estate, while Annie and Joy split the other half? Or does each receive a third of the estate?
The second reason to use the Oxford comma is it indicates a pause that mimics speech. When I write scripts with narration or dialogue, I include the final comma. It's hard to read "invitations to Pat, John and Martha" as punctuated; my voice wants to put in that extra comma. Omitting it affects meaning as well. Do John and Martha receive a joint invitation or separate ones?
The Oxford comma clarifies meaning and matches the rhythm of human speech. For those reasons, I'm a dedicated fan. You too can be a fan and give this punctuation mark some love. The Oxford comma has its own Facebook page.