How to stick oreo biscuits together to make a tower?
I'd recommend that unless you're just stacking oreos on top of each other, that you use something else for structure and then afix the oreos on using icing as a mastic.

Typically, you'd use one of the following:


A hollow structure made of gingerbread sheets, fastened together
A bit of dense cake (eg, pound cake), cut down to the proper size
Rice crispie treats, formed into the size you wanted


Personally, I'd go with the rice crispie treats, in part because you can just use that without any oreos.  (you can either take a handle of a spoon or similar and press in the texture you want once it's mostly but not fully set).

... but you can also deal with the texturing on the fondant ... just roll the fondant out a bit thick, apply it, and then use something appropriately sized and smooth (no sharp edges) to press in the grout lines.  If you have clay working tools, use those, but also the back edge of some table knives will work.   You have a longer working time than trying to do this on rice crispie treats, you just have to be more careful about not tearing or cutting the fondant.