We know how much creme goes into Double-Stuf Oreos (1.86 times the normal amount), and how long to dunk an Oreo to reach the optimal milk-to-cookie ratio (about 3 seconds), but the cookie's recipe remains a mystery. Nabisco keeps the formula a secret, but there is one thing we know for certain: The "creme" in Oreo filling isn't made of cream at all.
According to Extra Crispy, the white stuff sandwiched at the center of Oreo cookies achieves its creamy goodness without any dairy. Though the exact recipe has never been revealed, you can deduce from the ingredients list on a package of Oreos that the creme is made from some combination of sugar, high fructose corn syrup, soy lecithin, artificial flavor, and palm and/or canola oil. Many commercial white frostings are made from basically the same ingredients. Because the filling doesn't contain any actual cream, the FDA requires that Oreos be labeled "creme"-filled instead of cream-filled.
So does the lack of cream or butter make the world's best-selling cookie accidentally vegan? It's complicated. While they've always been dairy-free, Oreos originally contained pig lard. The manufacturers got rid of the ingredient in 1997, making the cookies both kosher and vegetarian.
As for whether or not they're vegan, Nabisco's official statement on the matter is "Oreos have milk as cross-contact and therefore are not suitable for vegans." Oreos are made in factories that also process dairy, so even if there's no milk in the recipe, trace amounts may contaminate the cookies. That means anyone who can't—or doesn't want to—ingest dairy under any circumstances should avoid them, but if you're less strict with your veganism, there's a whole world of Oreos out there for you to try.
[h/t Extra Crispy]