"I ate ice cream and mashed potatoes and gravy" (gross to eat them all as one dish) vs. "I ate ice cream, mashed potatoes and gravy" (the ice cream is clearly eaten separate from the mashed potatoes and gravy).
Is this really an example of use of the Oxford comma? If not, what would you call it?
No, that isn't an example of the Oxford comma. For that, place a comma before the conjunction:
 I ate ice cream, mashed potatoes, and gravy.
Birchy said
"I ate ice cream and mashed potatoes and gravy" (gross to eat them all as one dish) vs. "I ate ice cream, mashed potatoes and gravy" (the ice cream is clearly eaten separate from the mashed potatoes and gravy).Is this really an example of use of the Oxford comma? If not, what would you call it?
Peano's answer is correct, but I think you are asking a different question from what you wrote.
I think you are asking if an oxford comma would ever be acceptable in this sentence. Even for people who believe that an oxford comma should always be used.Â
The answer is that it depends on how you view mashed potatoes and gravy. If you view it as a single dish, then a comma should never separate them. But if you view them as two separate items, then an oxford comma would be appropriate. (If you believe oxford commas are ever appropriate.)
Please forgive me if I am assuming too much and you really meant exactly what you said.
Assuming (and I do) that mashed potatoes and gravy is consumed as a unit, I think the first example, with no commas and two ands, is the clearest expression of the meaning of the sentence. The second example, with one comma, would  either be equivalent to "I ate mashed potatoes and gravy, ice cream." or it would be the non-use of the Oxford comma in considering this to be a three-part meal, rather than two. Using the Oxford comma would clearly indicate the three-part meal. I am a strong advocate of the Oxford comma, because in my brain it almost invariably helps to clarify things, but as I interpret this sentence it is not a place to use it.
Welcome, Birchy.
tromboniator said: I am a strong advocate of the Oxford comma, because in my brain it almost invariably helps to clarify things ...
[So if "tromboniator" is at the beginning of a sentence, should I capitalize it? 
]
I tend to default to using the Oxford comma too. In some cases it's essential to eliminate ambiguity. The classic example I often see online is the dedication:Â To my parents, Ayn Rand and God. But if I spoke that dedication, knowing the meaning, I'd invariably insert a pause after "Rand" to avoid the confusion.
In some cases, it is indeed superfluous. An example of that is: I packed a tent, sleeping bag, tarp and gun.
In other cases, it can actually create ambiguity. Here's a great example: They went to Oregon with Betty, a maid, and a cook. Now that's just plain careless writing, since whatever is really meant, the sentence could be rewritten more clearly.
So even though I default to using the Oxford comma, I look at the sentence carefully, pronounce it aloud, and then decide to comma or not. If I find myself pausing when I speak the sentence I'll usually put the comma in there.