What is the 'Oxford comma'? It's the optional comma before the word 'and' at the end of a list, as in
He fell for her hook, line, and sinker
It's known as the Oxford comma because it was and is used by editors and printers at the Oxford University Press. It's also known as the 'serial comma' and while it isn't universally approved of, it's important when clarifying what might otherwise be an ambiguous list. Consider this apocryphal dedication to a book (quoted by the science fiction writer Teresa Nielsen Hayden):
To my parents, Ayn Rand and God
Or this alarming description of a Peter Ustinov television documentary:
[H]ighlights of his global tour include encounters with Nelson Mandela, an 800-year-old demigod and a dildo collector.
The ever-reliable Wikipedia offers the following example by way of further explanation:
They went to Oregon with Betty, a maid, and a cook.
This is ambiguous because it's unclear whether "a maid" is an appositive describing Betty, or the second in a list of three people. On the other hand, removing the final comma produces:
They went to Oregon with Betty, a maid and a cook.
This opens the possibility that Betty is both a maid and a cook (with "a maid and a cook" read as a unit, in apposition to Betty). So in this case neither the Oxford comma style nor the non-Oxford comma style resolves the ambiguity. A writer who intends a list of three distinct people (Betty, maid, cook) may create an ambiguous sentence, regardless of whether the serial comma is adopted. Furthermore, if the reader is unaware of which convention is being used, both versions are always ambiguous.
If Betty is one person:
They went to Oregon with Betty, who was a maid and a cook.
They went to Oregon with Betty, both a maid and a cook.
They went to Oregon with Betty, their maid and cook.
If Betty is one of two persons:
They went to Oregon with Betty (a maid) and a cook.
They went to Oregon with Betty—a maid—and a cook.
They went to Oregon with Betty, a maid, and with a cook.
They went to Oregon with the maid Betty and a cook.
They went to Oregon with a cook and Betty, a maid.
They went to Oregon with Betty, a maid; and a cook.
If Betty is one of three persons:
They went to Oregon with Betty, as well as a maid and a cook.
They went to Oregon with Betty and a maid and a cook.
They went to Oregon with Betty, one maid and a cook.
They went to Oregon with a maid, a cook, and Betty.
They went with Betty to Oregon with a maid and a cook.
What interests me most is when all this happened. It doesn't seem a particularly modern thing to do, does it? Going to Oregon I mean, and with an entourage. Whether or not Betty (not a modern name, Betty) is one, two, or three persons and whether or not she is a maid and/or a cook, one has the initial impression that the subject of the sentence is the pre-Depression American bourgeoisie, the kind of nuclear middle-class family who had, and travelled with, a servant, or servants. I suppose Betty could be a baby, or a young child, and perhaps sickly. Or she could equally be a mad older relation who needs constant care and attention. Perhaps the maid, if there is a maid who isn't Betty, was also her carer. The cook, employed to meet the capricious needs of an ailing child or deranged and possibly violent cousin, might have found Oregon a challenge, coming as the hypothetically wealthy family did from, say, New York's Upper East Side. I have the impression that the trip to Oregon wasn't a vacation (the Hamptons or Martha's Vineayrd would have been an obvious choice, or a resort in the Catskills if they were Jewish), so perhaps the move had more to do with business. Oregon is lumber country, but I don't think the head of this family group (if it is a family group) is a lumberjack. Whoever heard of a lumberjack with a servant, or servants?
Or what if 'they' are any number of children, and Betty their nanny? What if Betty isn't human? A dog perhaps? Or marmoset? Why Oregon?