Generously spray an 8x8 square baking pan with cooking spray. Set aside.
Use a food processor to coarsely chop the cookies. In the end you should have around six cups of Oreos.
In a large bowl, add butter and marshmallows. Microwave on high for 60 seconds. Then continue to microwave at 20 second intervals until all marshmallows are puffy and soft. Stir with a wooden spoon to mix up the butter and marshmallows.
Pour the broken up cookies into the bowl and stir well to evenly coat with the marshmallows.
Press into prepared pan using a sandwich bag.
Add sprinkles to the top and set aside to set up. You can also refrigerate.
To remove from pan, slide a knife or spatula on the side and lift up, the entire brick should pop out of the pan. Cut into triangles.
Notes
One package of Fruity Crisp Oreos has 21 cookies, so you'll need 2 packages (and will have some left over to snack on).If you don't have a food processor, you can use a plastic sandwich bag and rolling pin to crush the cookies. It might take a minute, but you can get the cookies down to pretty small crumbs.It took me 1:40 to microwave the marshmallows, just for a time reference.My "secret trick" to press the bars into the pan is to cover my hand with a sandwich bag that is coated with cooking spray. I use my hand to press the bars firmly.