Spray a 9"x13" pan generously with cooking spray, or grease with butter.
Put your Oreos (25 cookies) in a Ziploc bag and seal the bag, getting as much air out as possible. Using a rolling pin or your hands, bang on the bag until the Oreos are crushed into large chunks. You can choose how small you want the pieces to be, but I don't recommend using a food processor for this since it will crush the Oreos too finely.
In a large bowl (like the largest you own, these Rice Krispies like to fly around as you mix), mix the crushed Oreos and Rice Krispies (6 cups).
Add the mini marshmallows (8 cups) and butter (4 tablespoons) to a medium-large sauce pan. Turn your stove on to medium heat and stir the marshmallows and butter constantly until it's melted and bubbling. You really don't want to leave the mixture for more than a few seconds without stirring or the marshmallows on the bottom of the pan will burn.
Pour the marshmallow-butter mixture into the Oreo-Rice Krispies mixture to combine. I like to pour in a little of the marshmallow mixture in, then mix that in, then pour a little more in, then mix that in, and repeat. This helps prevent the Rice Krispies at the top from being too sticky and not being able to distribute the marshmallow all the way to the bottom of the bowl.
Pour the mixture into your prepared 9"x13" pan and gently pat it down to fill the pan in an even layer. You don't want to push it down forcefully or your Rice Krispie Treats will be really hard to bite into. If you're having a hard time patting it down because it's sticking to your hand, spray your hand with cooking spray to prevent sticking.
Let the mixture cool in the pan until it reaches room temperature. Then cut into pieces and enjoy!