Preheat the oven to 350 degrees F (175 degrees C).

Combine flour, brown sugar, oats, melted butter, and cinnamon in a bowl; stir until crumbly. Press 1/2 oat mixture into a 9x13-inch baking dish; cover with sliced apples.

Combine white sugar, water, cornstarch, and vanilla extract in a medium saucepan over medium heat; cook and stir until thick and clear, about 10 minutes. Pour over sliced apples; top with remaining 1/2 oat mixture.

Bake in the preheated oven until bubbly and golden, about 45 minutes.