Prepare a large baking sheet by lining it with parchment paper or a silicone baking mat.

In a microwave-safe bowl, melt white chocolate or candy melts in 30-second intervals until fully melted and smooth.

Dip the bottom half of each Oreo into the melted chocolate, and use a fork to lift them out, allowing excess chocolate to drip off.

Allow the dipped Oreos to set at room temperature for about 15 minutes, or refrigerate for quicker setting.

Melt the green candy melts, transfer to a piping bag, and pipe festive designs onto the Oreos.

Sprinkle rainbow nonpareils and other festive toppings before the chocolate fully sets. Let them set for 30 minutes.