What it is
The so-called Oxford Comma exemplifies how listing things has changed over the last 100 years or so. Most every modern style guide punctuates a run-of-the-mill list like this:
I bought eggs, ham and bread.
No so if you publish with the Oxford University Press. This august publisher has its own (antiquated) way of punctuating a simple list, and is sticking with it:
I bought eggs, ham, and bread.
This punctuation style is more a curiosity than a bona fide controversy, although Frank Kerouac and William Borroughs (both drunk, etc.) came to blows over whether or not this was the best usage. See the plaque commemorating this too typical example writers behaving badly.
H.L. Mencken summed up his strenuously American attitude toward the Oxford Comma in this parodic sentence: “There is something prissy, pedantic, and altogether un-American about the extra comma.”
There are exceptions to the rule (no Oxford Comma), such as extended lists containing compound elements:
I love grits, bacon and eggs, and orange juice.
Also, a comma can signal an apposition, which changes meaning by a long shot:
This book is dedicated to my parents, Ayn Rand and God
Or, to dial down the hubris, an Oxford Comma would include the writer’s real parents in the dedication:
This book is dedicated to my parents, Ayn Rand, and God
The simple rule is to use commas to compile a list of words or phrases according how they may or may not aid a clear, uncluttered communication of the words’ relation to each other.
*The American Language, 1921.Google+