Minor gross-out warning.
Many kinds of foods have coloring agents, and few of these have any noticeable impact on one's bowel movements. But something about the coloration used to make the cookies in Oreos so dark has the same effect on what one produces at the toilet after eating them, even if one has only had a few Oreos after eating a lot of other foods. The same isn't true of other similarly-colored foods, even other cookies.
In fact, there's only one other thing that has the same effect, and it isn't just not the same color as Oreos, it's decidedly not, since it's vivid, eye-searingly pink. That's Pepto Bismol. It does, inexplicably, the same thing to bowel movements as Oreos do.
Kind of makes Oreos seem a little more sinister, somehow.