Prepare the crust: In a bowl, mix the crushed Oreos and melted butter until combined. Press this mixture into the bottom of a greased 9x9-inch baking pan to form an even layer.
Make the mint fudge layer: In a separate bowl, beat together the powdered sugar, sweetened condensed milk, peppermint extract, and (if using) a few drops of green food coloring until smooth. Spread this mixture over the Oreo crust evenly.
Melt the chocolate: In a microwave-safe bowl, add chocolate chips and vegetable oil. Microwave in 30-second intervals, stirring in between, until completely melted and smooth.
Pour the chocolate over: Drizzle the melted chocolate on top of the mint fudge layer, spreading it evenly. Tap the pan lightly on the counter to remove any air bubbles and help the layers settle together.
Chill and serve: Cover the pan with plastic wrap and refrigerate for at least 2 hours or until firm. Once set, cut into squares and enjoy!
