Preheat oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper.
In a large bowl, combine flour, oats, sugar, baking powder, and salt.
Cut in the cold butter using a pastry cutter or your fingers until crumbly.
Stir in vanilla extract.
Press two-thirds of the mixture firmly into the prepared pan to form the crust.
Bake for 15 minutes, then remove from oven.
In another bowl, mix apples, lemon juice, brown sugar, cornstarch, cinnamon, and nutmeg.
Spread apple mixture evenly over the warm crust.
Drizzle caramel sauce over the apples.
Sprinkle remaining crumb mixture on top.
Bake for 30–35 minutes, until golden and bubbly.
Let cool completely before slicing into bars.