Preheat the oven to 350 degrees Fahrenheit.
Line three 6 inch pans or two 9 inch pans with parchment paper and set aside. In a large mixing bowl or bowl of a stand mixer, add the softened/room temperature butter and sugar and beat together for about 1 minute on medium speed.
Add in the remaining wet ingredients and beat together until well combined.
Add in the dry ingredients, aside from the oreos, making sure to spoon and level the flour, don't scoop right from the bag. Beat together until just combined.
Chop up the oreos, leaving some larger pieces and some smaller and add to the batter.
Fold in using a rubber spatula and mix until combined.
Add to your prepared pans, trying to keep each pan as even as possible.
For 6 inch pans, bake for 38-44 minutes, for 9 inch pans, bake for 25-35 minutes or until a toothpick comes out clean.
Remove from the oven and let cool in the pan for about 15 minutes, then flip onto a wire rack and let cool completely.
To make the buttercream, add the room temperature butter to the bowl of a stand mixer and beat for about 2-3 minutes or until fluffy.
Slowly add in the sugar and let run for about 5 minutes until fluffy. Add in milk and vanilla and mix until combined.
Add in the crushed oreos and cream together for about 20 seconds.
Once the cake has cooled completely, frost in between each layer, then around the full cake. Decorate as desired and enjoy!