Combine all ingredients except the Oreo into a microwave-safe mug. If you want the cake just brimming over the top, use an 8 oz mug. If you use a larger mug, that is fine, your cake just won't rise as high. If you want the brimming over the top of the mug look for bigger mugs, you can always 1.5x or 2x the recipe. Mix ingredients with a small whisk until batter is smooth.
Add in Oreo. Using a fork or your whisk, crush Oreo into batter until only small pieces remain. Stir crushed cookie evenly into batter.
Cook in microwave for about 1 minute (This is the cooking time for my 1000 watt microwave). Let cake cool a few minutes before eating. Cake is best consumed while still warm or within a few hours of it being cooked.