Melt the white candy melts according to package instructions until smooth and creamy.
Melt the green candy melts in a separate bowl until smooth.
Dip the bottom of each Oreo into the white candy melts so only the bottom is coated. Gently tap the fork on the bowl edge to remove excess.
Place the coated Oreos on a baking sheet or plate.
Transfer the green candy melts to a piping bag.
Drizzle the green candy melts over the top of each Oreo in a zigzag pattern to make it look like a Christmas tree.
While the green coating is still wet, place a star sprinkle on top of each tree.
Sprinkle the nonpareils over the trees to decorate.
Let the cookies set completely. For faster setting, place the baking sheet or plate in the refrigerator for about 10 minutes.
Once set, carefully remove the Christmas tree Oreos from the surface.