The oxford comma does not guarantee a list to be unambiguous. [0]
> But the serial comma can also create ambiguity. Consider the following adjusted version of the dedication [discussed in the preceding paragraph]: To my mother, Ayn Rand [,] and God. With the serial comma, the reader could understand the dedication as meaning either that the book is dedicated three ways or that the book is dedicated to the writer’s mother, who happens to be Ayn Rand, and to God. Omitting the serial comma makes the latter meaning less likely.
That's not a particularly good example, since you have the same ambiguity between the use of commas to set off non-restrictive appositives and the use of commas to separate list items, it still occurs in lists where the ambiguity is between four (or more) elements with no appositive or three (or more) elements, one of which has a non-restrictive appositive attached.
You can't really eliminate that source of potential ambiguity in isolated excerpts, only in larger works or bodies of work by adopting consistent practices that go beyond whether or not you use the Oxford comma (mostly, structuring sentences so that if one or more items in a list needs a non-restrictive appositive, it is either just the last item -- which is already set off with a conjunction whether or not it also has the Oxford comma preceding it, so the following appositive is unambiguously not a list item) or using parentheses rather than commas for non-restrictive appositives attached to list items (which is clearer even for two item lists, where item-separating commas, Oxford or otherwise, don't actually come into play.)
> But the serial comma can also create ambiguity. Consider the following adjusted version of the dedication [discussed in the preceding paragraph]: To my mother, Ayn Rand [,] and God. With the serial comma, the reader could understand the dedication as meaning either that the book is dedicated three ways or that the book is dedicated to the writer’s mother, who happens to be Ayn Rand, and to God. Omitting the serial comma makes the latter meaning less likely.
[0]: http://www.adamsdrafting.com/the-serial-comma-can-cause-ambi...