In a large bowl, combine the flour, brown sugar, oats and butter; set aside 1 cup for topping. Press remaining crumb mixture into an ungreased 9-in. pie plate; set aside.
For filling, combine the sugar, cornstarch and water in a large saucepan until smooth; bring to a boil. Cook and stir for 2 minutes or until thickened. Remove from the heat; stir in apples and vanilla.
Pour into crust; top with reserved crumb mixture. Bake at 350° for 40-45 minutes or until crust is golden brown. Cool on a wire rack.