Melt the white candy melts according to the package instructions until smooth and creamy.
Dip the bottom of each Oreo into the white candy melts, ensuring it's fully coated at the bottom only. Gently tap the fork on the edge of the bowl to remove excess candy melt.
Place the coated Oreos onto a baking sheet lined with parchment paper.
Melt the green candy melts in a separate bowl, also until smooth.
Transfer the melted green candy melts to a piping bag. Drizzle the green candy melts over the top of each Oreo in a zigzag pattern to create the look of a Christmas tree.
While the green candy coating is still wet, place a star sprinkle at the top of each tree. Sprinkle the nonpareils sprinkles onto the trees to decorate.
Allow the cookies to set completely. You can speed up this process by placing the baking sheet or plate in the refrigerator for about 10 minutes.
Notes
Storage: Store the Christmas tree Oreos in an airtight container at room temperature for up to one week.