Crush 4 of the Oreo cookies into small pieces directly in a large microwave-safe mug.
Add milk, sugar, and baking powder to the mug with the crushed Oreos. Stir until the mixture is well combined, then break the last Oreo into a few larger chunks and fold it into the batter.
Microwave on high for 1-2 minutes, checking on it after 1 minute and every 15 seconds after that until done.
Serve warm, topped with ice cream or whipped cream if desired.