Preheat oven to 350°F (175°C) and grease an 8x8 inch square baking pan.
In a large mixing bowl, combine the oats, flour, brown sugar, baking soda, and salt.
Pour the melted butter into the oat mixture and stir until evenly moistened and crumbly.
Press exactly half of the oat mixture into the bottom of the prepared pan to form a level crust.
In a medium bowl, toss the diced apples with granulated sugar, cinnamon, and lemon juice until coated.
Spread the apple mixture evenly over the bottom crust.
Sprinkle the remaining oat mixture over the top of the apples and press down lightly with a spatula.
Bake for 30 to 35 minutes until the topping is golden brown and the apples are tender.
Remove from the oven and allow to cool completely in the pan before slicing into 12 rectangular bars.