Put white chocolate chips in a glass bowl and place in microwave. Microwave on high for 40 seconds. Stir chocolate with rubber spatula. Place back in microwave and cook on high for 20 seconds. Stir. Continue microwaving for 20 seconds and stirring until chocolate is fully melted. Alternatively, you can melt chocolate over a double broiler.
On a waxed paper-lined cookie sheet, spread out about ¾ of the crumbled Oreo Cookies, reserving the remaining ¼. When chocolate is melted, pour over the Oreos on the cookie sheet and spread in an even layer over the cookies.
Sprinkle remaining cookies, M&Ms, and sprinkles over the chocolate.
Let bark rest for about 90 minutes before breaking into pieces. If you want it ready faster, place cookie sheet in fridge for about 30 minutes.
Store bark in an airtight container for up to two weeks (if it lasts that long!).