Preheat your oven to 350°F (175°C) and grease your 9x13 inch baking pan or line it with parchment paper for easy removal.
In a large mixing bowl, combine the melted butter, egg, light brown sugar, and vanilla extract. Whisk until smooth and well combined.
Gradually add the all-purpose flour to the wet mixture, stirring gently until just combined. Be careful not to overmix!
Gently fold in the chopped Oreo cookies and M&Ms, ensuring they are evenly distributed throughout the batter.
Pour the batter into the prepared baking pan and use a spatula to spread it evenly across the pan.
Bake in the preheated oven for 20-25 minutes, or until the edges are golden brown and a toothpick inserted in the center comes out clean.
Allow the bars to cool in the pan for about 10-15 minutes before transferring them to a wire rack to cool completely. Once cooled, slice into squares and enjoy!