Line a large baking sheet with parchment paper and place Oreo cookies nearby.
Melt the white chocolate or candy melts in a microwave-safe bowl in 30-second intervals until smooth.
Dip the bottom half of each Oreo into the melted chocolate and place on the prepared baking sheet. Let set at room temperature for about 15 minutes.
Melt the green candy melts using the same method and transfer to a piping bag.
Pipe a zig-zag line down the center of each dipped Oreo to resemble a Christmas tree.
Immediately sprinkle rainbow nonpareils over the piped icing and add optional decorative elements if desired.
Let the decorated Christmas Oreos sit until hardened, about 30 minutes, or refrigerate for faster setting.