Use your cake leveler to slice off the uneven tops of the cake layers.
Pipe a ring of buttercream around the outside of the first layer, to create a dam. Then, fill the inside with the chocolate orange ganache. On top of that, carefully place one of your Oreo discs. Then, carefully smooth a small amount of buttercream on top. Then, add a second layer of cake, and repeat the process. For a visual explanation of stacking and layering a cake, watch the video via the link in the notes section below.
Using an offset spatula or piping bag, coat the outside of the cake in a thick layer of buttercream. Then, hold the cake scraper at a tight angle to the cake and smooth the sides. This first layer doesn't have to be perfect (and it likely won't be.) You just want to lock in all of the layers and crumbs. When it is smoothed to your liking, place the cake in the refrigerator for 10-15 minutes to harden.
Once the crumb coat has hardened to the touch, recoat the cake in a similar manner with another smooth layer of buttercream. This time, pay careful attention to smooth it out neatly. Use an offset spatula to sweep any remaining buttercream off of the top edges and onto the center of the top of the cake to create a clean, sharp edge. Chill the cake again, for 10-15 minutes.
Use a palette knife or piping bag to add any type of decoration you prefer to the chilled cake. See the notes below for a link to my more thorough palette knife decorating tutorial. Then, before adding your ganache drip, make sure to chill the cake once more. A cold cake and slightly warm ganache create the perfect drip. Use a piping bag with a round tip to slowly pipe along the top edge of the cake, applying more pressure where you want a drip to form. Start on the back of the cake for some practice. If the drips are too thin and run down the cake, let your ganache cool more. If the drips barely move, heat the ganache slightly to help loosen it up. You can watch how I add a ganache drip via the link in the notes below. Once the ganache is dried, you can very gently add some luster dust to the drips with a food-safe paintbrush. I didn't use any liquid or alcohol to make the dust viscous, because of the fragility of the drips, I just added the dust directly as is to the chocolate.