-LSB-...] usually when one frosts a cake, especially the non-smooth sides and underside, s / he starts with a crumb coat, a coating of frosting that catches the stray crumbs and eventually gets hidden under a second -LSB-...]
I put a layer of parchment paper around the base of my cake to catch the crumbs, and used a knife to lift the crushed Oreos onto the sides of the cake.