In a medium bowl, mix the oatmeal, brown sugar, flour, cinnamon and butter until a crumbly mixture forms. Spread half of this mixture into the bottom of a greased 9x13 pan and press down firmly to create a crust.
Peel and slice your apples into and extra large bowl with cold water and lemon juice to prevent the apples from browning.
In a medium saucepan, whisk together the sugar, cornstarch, water and vanilla. Cook over high heat until it is thickened. Remove from the burner and stir in the peanut butter until smooth.
Drain the water and lemon juice off of the apples and pour the peanut butter mixture over them. Stir until all of the apples are well coated. Spread the apples over the oatmeal crust.
Sprinkle remainder of oatmeal mixture over the apples.
Bake at 350° for 1 hour. Let cool to room temperature before serving.
Store leftovers refrigerated.