Preheat oven to 350 and place cupcake liners in pan.
Mix oil, water, eggs, and chocolate cupcake mix according to the package directions. Bake as directed, then move to a cooling rack.
Beat unsalted butter in a stand mixer for 3 minutes until light and fluffy. Add in confectioners sugar, cocoa powder, salt, vanilla, and milk until well combined.
Crush Oreo cookies in a plastic bag using a rolling pin. Set cookie crumbs aside until ready to decorate.
Melt chocolate chips or wafers for 30 seconds at a time in the microwave, stirring frequently. Transfer melted chocolate into a piping bag. Pipe chocolate into the shape of a tree on a lined baking sheet. Press the leaf sprinkles into the chocolate. Refrigerate until the chocolate is firm. Carefully peel the chocolate tree off the parchment paper.
Pipe frosting on cupcakes. Decorate with Oreo cookie crumbs, chocolate tree, leaf sprinkles, and an icing pumpkin. Enjoy!