Line a medium baking sheet with parchment paper. Set aside.
Optional: Crumble the oreos into small pieces with your fingers and set aside.
Add the semisweet dark chocolate and the peppermint oil to a microwave-safe bowl. Microwave in 20-second intervals until fully melted, stirring each time (about 3-4 minutes total).**
Pour the chocolate onto the middle of the prepared baking sheet. Spread with a spatula to about ¼-inch (1/2 cm) thick. If you're using the crumbled oreos, sprinkle them evenly on top.
Chill for 15 minutes while you prepare the remaining ingredients.
Place the candy canes into a ziptop bag. Seal and pound lightly with a rolling pin to make small chunks (but don't pulverize completely). Set aside.
Place the white chocolate in a medium-sized bowl. Use the same method above to microwave until melted.**
Remove the baking sheet from the fridge. Pour the white chocolate evenly across the semisweet chocolate, spreading with a spatula to cover. Then sprinkle the crushed candy cane on top.
Let it sit at room temperature for 1 hour or more until completely solid. If you're in a hurry, you can place it in the fridge to chill for 15 minutes, and then let it sit on the counter for 10 minutes before proceeding.
Use the parchment to lift the bark out of the baking sheet and onto a cutting board. Using plastic wrap or rubber gloves, hold the bark with your fingers and snap into pieces. You can also use a large, sharp knife to cut it into pieces of desired size.