Prepare the pan by lining it with parchment paper or foil, leaving an inch or two on the sides so you can easily lift the fudge out when it has set.
(For pictures, I used an 8x8 pan so the fudge would be thicker, and I can cut into 1x1 inch squares. I have made this fudge before and used a 9x13 cake pan, which makes the fudge thinner, so you can cut smaller pieces. It’s just a personal preference.)
Once the pan is ready, set it aside while making the fudge.
To begin the fudge, add the sugar, butter, and milk to a medium to a large saucepan. Place on the stovetop using medium heat. It will begin to thicken and boil, pulling from the sides of the pan.
Continue stirring until it slowly begins to boil into a foam. (About 3-4 minutes) Once it has a thick mixture with a foam appearance, remove it from heat and stir in the white chips and marshmallows, stirring until they are completely melted.
The fudge will begin to thicken slightly as you are stirring. Quickly stir in the crumbled Oreos, making sure they are evenly distributed into the fudge mixture. (I left out a handful of the oreo crumble to sprinkle on top of the fudge, but you can add it all if you prefer. )
Pour the mixed fudge into the prepared pan, and using a rubber spatula or wooden spoon, spread it evenly into the pan. Allow the fudge to sit at room temperature to cool so it can set. If you make the fudge ahead of time, be sure to cover and keep it in an airtight container, so it doesn’t dry out.