Preheat oven to 350°. In a large bowl, combine oats, brown sugar, flour and salt; stir well. Cut in butter until crumbly. Pat 2 cups of mixture into a greased 13x9-in. baking dish; cover with rhubarb. , In a saucepan, combine sugar and cornstarch. Stir in water; cook until mixture is thickened and clear. Stir in extract and cherry filling; spoon over rhubarb. Combine nuts with reserved crumb mixture; sprinkle over cherries. Bake until filling is bubbly and topping is lightly browned, 40-45 minutes. If desired, serve with ice cream.