Preheat the oven to 350 degrees F (175 degrees C). Grease cookie sheets.
Beat brown sugar, white sugar, margarine, and butter together in a very large bowl until smooth. Beat in eggs, one at a time, then stir in vanilla. Whisk flour, oats, baking soda, baking powder, and salt together in a separate bowl; stir into sugar mixture until just combined. Stir in chocolate chips and candy coated chocolate pieces. Drop by tablespoonfuls onto the prepared cookie sheets.
Bake in the preheated oven until edges are golden, 10 to 11 minutes (for chewy cookies, take them out before they look done).
Linda's Monster Cookies · Dinner Match Lab | Dinner Match Lab