Displayed prominantly on the Oreo package in my freezer (more on this later) is the phrase "America's Favorite Cookie". Some people might consider this to be a fairly hefty claim but it turns out to be statistically justifiable, with Oreos selling roughly 10 times as well as the next most popular cookie. Quite frankly, I'm not surprised as I consider the Oreo to be the all around finest cookie on the market. We (Dan and I) have been trying to keep at least one package of Oreos in our freezer at all times for quite a while and they've essentially become a staple around here, with us going through approximately one package every week or two.
Now, as to why we keep our Oreos in the freezer; they're better that way. Keeping Oreos in the freezer makes them stay fresh a whole lot longer than if they aren't kept in the freezer (even if you don't keep them in a resealable package). Also, frozen Oreos are a bit stiffer, harder and more solid, which I rather like.
So yeah, Oreos are great and the rest of America agrees. Also, if you look at some of the other statistics, Nabisco has a stangle-hold on cookies and crackers in the US.