Line a large baking sheet with parchment paper or a silicone mat. Set aside.
Place white chocolate in a microwave-safe bowl and melt in 20-second intervals, stirring after each burst, until completely smooth and glossy. This takes about 1-2 minutes total.
If using vanilla extract, stir it into the melted white chocolate until combined.
Reserve about 1/4 cup of crushed Oreos for topping. Fold the remaining crushed cookies into the white chocolate, stirring gently until evenly distributed.
Pour the mixture onto the prepared baking sheet and spread into an even layer about 1/4-inch thick using an offset spatula or the back of a spoon.
Sprinkle the reserved Oreo pieces over the top, pressing them lightly into the surface.
Drizzle melted dark chocolate over the bark in a back-and-forth motion using a spoon or piping bag.
Refrigerate for 20-25 minutes until completely firm and set.
Remove from refrigerator and let stand at room temperature for 5 minutes. Break into irregular pieces by hand or cut with a sharp knife.
