Place the butter in a 9x13-inch baking dish (or an 8x10-inch dish for thicker bars). Place the pan in the oven just until the butter melts, then remove it. Alternatively, grease the pan with nonstick cooking spray.
Crush about 20 Oreo cookies into fine crumbs using a food processor or by placing them in a zip-top bag and crushing with a rolling pin.
Sprinkle the Oreo crumbs evenly over the melted butter and press into an even crust.
Evenly sprinkle the chocolate chips and butterscotch chips over the crust.
Pour the sweetened condensed milk evenly over the top.
Roughly chop or crush the remaining 5-6 Oreo cookies into small chunks and sprinkle them over the bars.
Bake for 20 minutes, or until the edges are lightly browned and the center is bubbly.
Let the bars cool completely before slicing into squares and serving.
Notes
You are going to be so glad you made these tasty magic bars!