In a large mixing bowl, stir together oats, flour, brown sugar, M&M's, and baking soda. Stir in melted butter.
Remove 1 cup of oatmeal mixture and set aside. Press remaining mix into a greased 9 x 13 inch baking pan.
Prepare brownie mix according to package directions using the oil, water, and eggs, and spread over oatmeal layer. Sprinkle clumps of reserved oatmeal mixture over top of brownies.
Bake at 350 degrees for 25 - 30 minutes. Cool completely before serving.