• Beat butter and cream cheese together until smooth, add vanilla, powdered sugar and salt.  Beat until smooth.
• Orange Frosting: Remove 1/4 of the mixture into a separate bowl and tint orange with food coloring.  Set aside for piping later.
• Crumb Coat: Remove 1/4 of the mixture to an additional separate bowl.  Use this to make the crumb coat on the cake (this seals the crumbs in to make your final icing sit prettier on your cake.)
• Oreo Filling: Combine the roughly chopped Oreo pieces with the remaining 2/3 of the cream cheese buttercream.  Fold gently until cookie pieces are distributed throughout the filling.
• Chocolate Swiss Meringue Buttercream, see the recipe in this post.  I doubled the batch just to be sure but had quite a bit leftover.  (Not a problem in this house.)

Assembly:

• Place one chilled cake layer down on cake board or flat platter.  Cover evenly with Oreo Filling.  Top with cake layer #2, upside down so top is flat.  Cover with Crumb Coat Cream Cheese Buttercream, sealing in crumbs.  Chill if necessary.  (I freeze my layers so by the time my crumb coat is on, the chill from the layers sets the frosting nicely.)  Frost with Chocolate Swiss Meringue Buttercream, pipe decoratively, using a large open star tip if desired.  Using a large round piping tip, pipe small orange dots around cake and finish with an orange initial in the center of the cake.  Additional garnishes of sparkling sugar in white and sanding sugar in orange were used as well.  Chill cake until 1/2 hour before serving.  (The Chocolate Swiss Meringue Buttercream is fragile and best served slightly chilled.)
• Fill it.
  
• Beginning of crumb-coat, typically done with a portion of the final frosting, but cream cheese frosting is a bit sturdier than the Swiss meringue buttercream to really seal in the crumbs.
  
• Finished crumb coat.
  
• After final frosting and decorations.
• Had a little help from his cousins adding the final sprinkles.