Preheat the oven to 350F degrees. Line an 8x8 inch pan with aluminium foil and grease with non-stick cooking spray or butter.
In a medium-sized bowl add the oil and sugars. Heat in the microwave for 30 seconds, then stir together the rubber spatula or wooden spoon. Then stir in the cocoa powder, salt and vanilla extract. When the mixture is no longer hot, whisk in the eggs. Then finally stir in the flour using your rubber spatula or wooden spoon.
Spoon about ½ of the batter into the prepared pan, then place a layer of the 16 Oreo cookies over top. Then spoon the leftover brownie batter over the cookies and spread it evenly over top. If you aren't able to completely cover the cookies, it's ok.
Bake in the preheated oven for 22-24 minutes, or until the top looks set and the sides are just pulling away. Allow to cool fully in the pan.
Once the brownies are fully cooled, make the frosting. In a medium-sized bowl, beat together the butter and powdered sugar using an electric mixer on low speed. Once combined, turn the mixer to medium-speed and beat until fluffy. Then beat in the marshmallow cream. If the mixture is too thick, beat in 1 tablespoon of milk or cream
Remove the brownies from the pan by lifting them out by the foil. Then frost with the marshmallow frosting using a flat-edge knife and top with the additional crushed Oreo cookies.