You knew it was coming.
In a series of three or more items, is there a comma just before the conjunction?
Pick up a loaf of bread, a container of milk, and a stick of butter.
or
Pick up a loaf of bread, a container of milk and a stick of butter.
I am firmly in favor of the Oxford comma.
In a construction that may or may not contain an appositive such as My sister, a hairy ape, and a carnival clown walked the tightrope, I'd rewrite the sentence so that it's clear whether or not my sister is a hairy ape.