Melt the white chocolate and stir in green food colouring. In a separate bowl, melt the dark chocolate and add black food colouring as needed to match the Oreo colour.
This is the messy bit: dip Bugles in chocolate (or finger paint it on). Allow to cool.
Attach to the Oreo with melted chocolate. Allow to cool.
Assemble: pour the green-colour white chocolate into a puddle for the melted witch. My chocolate wasn't soft enough here. Quickly add the hat and make a broom out of a pretzel and hickory sticks.
Once completely cool, peel off the foil.
And try these Spooky Spiders in my separate guide!