“And what happens if Oreos changes their formula and puts actual dairy into the cookies? How will anybody know about the change?”
Why is this the OUs problem. They already label the cookies OUD. If you want to try and outsmart them and start klerring about each individual item and its certification, that is your problem, not theirs.
Listing dairy as an allergen and labeling something ou pareve, or just plain ou are not mutually exclusive. The allergen can potentially be one part in a million, but it is listed as a medical precaution, not a kashrus issue.