Question: According to the Wikipedia page for the [Oxford Comma](http://en.wikipedia.org/wiki/Serial_comma#Arguments_for_and_against), "Use of the comma is consistent with conventional practice" and "Use of the comma is inconsistent with conventional practice." Did the Oxford Comma come before its omission, or was the Oxford Comma traditionally omitted?

It makes logical sense that every item in a list would be separated in the same way: by a comma. If the Oxford Comma is conventionally correct, when and why did people begin to omit it?

Answer: 
I don't have information about the history of this, so I'm just responding to the second part of your question. Why should there be no comma before 'and'? The answer is simple - there has never been a convention for adding a comma (as far as I know) for mentioning two items: *I like apples and oranges* or *She doesn't mind running or swimming*. The problem comes when you want to add a third item: *I like pears, apples and oranges* or *She doesn't mind jogging, running or swimming*. It must seem fussy to people to insist on a comma before 'and' here, if there was no comma for two items.
According to Willam J. Strunk in *The Elements of Style*, Rule 2 states that a comma should be used after each term except the last in lists of three or more items. There is an exception which is in names of businesses, the correct example given is: Brown, Shipley and Company (as opposed to Brown, Shipley, and Company).

See the link for the full online text:

<https://faculty.washington.edu/heagerty/Courses/b572/public/StrunkWhite.pdf>