Heat oven to 350 degrees and spray 9x13 baking pan with cooking spray or line with parchemnt paper. You can also line with tin foil and spray the foil with the cooking spray.
In a large bowl combine the oats, flour, brown sugar, salt, melted butter, and chopped pecans. Stir together until it resembles "wet sand" and the mixture is crumbly.
Measure 1 1/2 cups of the crumble mixture and set it aside. Press the remaining crumble mixture from the bowl into the prepared baking pan. Press down to form the crust.
In a saucepan, over medium high heat, combine the sweetened condensed milk, chocolate chips, and butter. Stir and let it melt together. Once it's melted and combined together, take off the stove and pour the chocolate over the crust.
Press the reserved crumble over the chocolate. Add the m&m's and sprinkles, gently press them into the crumble.
Cook for 20-23 minutes. The edges will be slightly pulled away from the pan. Let cool for about 30 minutes to 1 hour so the bars can cool and come together.
Careful not to overbake these! It's hard to tell when they're done because of that thick crumble. But pull them out in the cook time and let them cool. They will continue to cook slightly more as they are cooling.
Use either milk chocolate chips or semi-sweet chocolate chips. Whichever you prefer. For a less sweet dessert use the semi-sweet. For more milk chocolate and sweet flavor, use the milk chocolate. I've used both and they both are good. Or use 1/2 a cup of each.
If you use unsalted butter you will want to add additional salt. Like an additional 1/4 teaspoon for a total of 3/4 teaspoon salt.