Preheat the oven to 350°F (175°C). Grease and line a 9x13 inch baking pan with parchment paper, leaving some overhang for easy removal later.
In a medium bowl, whisk together the flour, oats, baking soda, and salt. Set aside.
In a large mixing bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy.
Add the egg and vanilla extract to the butter mixture and mix until fully combined.
Gradually add the flour mixture to the wet ingredients and mix just until combined. Do not overmix.
Take half of the mixture (about 2 cups) and press it evenly into the bottom of the prepared baking pan to form the base layer.
In a separate bowl, toss the diced apples with cinnamon until well coated. Evenly distribute the apple mixture over the pressed crust.
Drizzle half of the caramel sauce over the apples, allowing it to seep into the layers.
Take the remaining oat mixture and sprinkle it over the apple and caramel layer for the topping.
Bake in the preheated oven for 25-30 minutes, until the top is golden brown and the apples are tender.
Once baked, remove from the oven and allow to cool in the pan for 15 minutes. Drizzle the remaining caramel sauce over the top and sprinkle with sea salt.
Allow the bars to cool completely in the pan before lifting them out using the parchment overhang. Cut into squares and serve.
Notes
Serve the bars on a wooden platter, drizzled with extra caramel and garnished with a sprinkle of sea salt for an appealing finish.