Prepare the Pan: Grease a 9x13 inch baking pan with cooking spray or butter. Set it aside.

Melt Butter and Marshmallows: In a large saucepan or pot, melt the butter over medium-low heat. Once melted, add the marshmallows to the saucepan. Stir continuously until the marshmallows have completely melted and combined with the butter.

Add Protein Powder: Once the marshmallows are melted, remove the saucepan from the heat. Add the cookies and cream protein powder to the marshmallow mixture. Stir well until the protein powder is fully incorporated and the mixture is smooth.

Mix in Cereal and Crushed Cookies: Add the crispy rice cereal to the marshmallow-protein mixture. Gently fold it in until all the cereal is coated evenly. Then, fold in the crushed Oreo cookies, making sure they are distributed throughout the mixture.

Press Into Pan: Transfer the mixture into the prepared baking pan. Use a spatula or greased hands to press the mixture firmly and evenly into the pan. Ensure it's compacted well.

Let it Set: Allow the cereal treats to cool and set at room temperature for about 30 minutes to an hour. You can also place them in the refrigerator for faster setting.

Cut and Serve: Once the treats have set, use a sharp knife to cut them into squares or rectangles of your desired size.