Commenters onRedditnote that the Costco variety don’t soften in milk.

Anotherthreadpoints out that they have a different texture, and just in general taste different to the regular kind.

Or, rather, there are two key differences.

Oreos

PerShop Oreo, classic Oreos list artificial flavoring among their ingredients.

The Costco variety lists natural flavoring instead (viaCostco).

Oreos