Line a 13" x 9" pan with parchment paper and lightly spray the parchment with non-stick cooking spray.

Place the 18-ounce package of Oreo cookies in a gallon-sized Ziploc bag and crush them with a rolling pin until broken into mostly small pieces with some larger chunks remaining. (Alternatively, break the cookies in a large bowl using a wooden spoon.)

Put the 2 bags (about 4 1/2 cups) white chocolate chips and the 2 tablespoons vegetable shortening in a large microwave-safe bowl.

Microwave the chips and shortening at 50% power for 30 seconds. Remove and stir. Continue microwaving at 50% power in 15-second increments, stirring after each increment, until the chips are mostly melted. Remove from the microwave and stir until completely smooth. (Total time will vary by microwave; avoid overheating.)

Add the crushed Oreo pieces to the melted white chocolate and gently fold until the cookie pieces are evenly coated and distributed.

Pour and spread the mixture evenly into the prepared 13" x 9" pan, using a spatula or the back of a spoon to level the surface.

Let the bark harden until firm — about a couple of hours at room temperature, or place the pan in the refrigerator to speed the process (check after 30–60 minutes).

When completely firm, lift the bark from the pan using the parchment paper and peel the parchment away. Use a sharp knife to cut the bark into pieces; if the knife sticks, warm it briefly under hot water, dry it, and then cut.