Preheat the oven to 350 degrees. Use convection bake if you have it.
In a medium mixing bowl, partially melt the butter in the microwave.
Add the rest of the dry ingredients to the melted butter (oats, flour, brown sugar, salt and cinnamon). Stir well with a spoon or silicone spatula until the ingredients are fully incorporated.
Line an 8x8 baking pan with parchment paper (or grease it well). Press half of the oat mixture evenly throughout the bottom of the pan to form the crust.
Set the crust and the other half of the oat mixture aside.
To make the apple filling and assemble the apple crisp bars
Pour the apples into a mixing bowl and add the rest of the filling ingredients (brown sugar, sugar, flour and cinnamon). Stir well with a spoon or silicone spatula until all of the apples have been evenly coated.
Pour the apple filling over the prepared oat crust and spread them evenly throughout the pan.
Sprinkle the remaining oat topping over the apple filling and bake for 45-60 minutes or until the apples are soft and bubbling (use a fork or knife to poke the apples to see if they are soft).