Preheat the oven to 375°F. Coat the muffin tin with cooking spray or butter (optional: muffin paper liner).
In a medium size bowl: combine the streusel ingredients (flour, sugar, butter & crushed Oreos). Mix well then set a side for later use.
In a separate large bowl: mix the sugar and eggs together well. Then add the milk, butter and vanilla extract, while continuing to mix. Once the mix has become uniform. Add in the flour, baking powder and salt. Then use a mixer to form the batter.
Add chopped Oreos to the batter mix. Use a spatula to fold until the Oreos are evenly distributed throughout (do not mix rapidly or use a mixer at this step. It is important to be very gentle at this stage).
Divide the batter into the 16 muffin tin cups. Sprinkle the streusel mixture atop of the batter.
Bake for 18-20 minutes or until a toothpick placed in the center comes out clean. Let the muffins cool in the tin for about 5 minutes and then transfer them to a baking rack until completely cooled.