Line an 8x10 baking pan with parchment paper for ease of removal or simply spray the pan with baking spray.
In a food processor crush the 4 cups worth of Oreo's for the base of the crust. If you do not have a food processor you can put the Oreo's in a zip locked bag and use a rolling pin to crush them. You want the Oreo's crushed to a small fine texture.
In a medium sized bowl, whisk together the crushed Oreos, melted butter and salt until the mixture resembles wet sand.
Press the crust into the bottom of the prepared baking pan.
Pour the can of sweetened condensed milk onto the crust and spread to the edges.
Then you will start to evenly sprinkle the remaining ingredients over the condensed milk. I like to start with the crushed Oreo's, then the chocolate chips, white chocolate chips, butterscotch chips and finally the M & M's.
Place the pan into the oven to bake for 45 minutes.
Once baked, remove from the oven and place onto a cooling rack to cool for at least 1 hour before slicing and serving.
A different way of baking these bars is to bake the Oreo crust with the sweetened condensed milk without the toppings for 25 minutes. Add the toppings and bake for another 20 minutes.