Line three baking sheets with parchment paper and set aside. Unwrap the Reese's peanut butter cups and place them large side down on one of the baking sheets.
Add an Oreo to the melted candy and use a rubber spatula to scoop the candy over the top of it. Use a fork to scoop under the Oreo and lift it out, tapping until all the excess candy falls off, then sliding the bottom of the fork against the bowl. Use a toothpick to help slide the Oreo off the fork onto the prepared baking sheets.
Then add the 2D icing decorations and additional sprinkles to decorate the candy-coated Oreos. Let the Oreos set completely, about 10 to 15 minutes after being dipped. Reheat the candy melts if needed for 15 to 30 seconds until smooth again.
Dip the bottoms of the Oreos about ¼-inch into the candy melts and then gently press them on top of the peanut butter cups, holding each one for 10 to 20 seconds to ensure it stays. Let set completely before enjoying, about 15 minutes.