CRUST...Combine dry cake mix, oats, walnuts and melted butter. Mix Well. Press 2 1/2 cups of crumble mixture firmly into bottom of greased 9x13 pan. Set aside remaining crumbs for topping.
FILLING...In a saucepan combine peaces, water and brown sugar. Simmer over low heat for 5 min, stirring occasionally. Mix cornstarch and lemon juice until smooth. Add to peaches. Cook stirring, until thickened. Pour over crust. Sprinkly reserved crumbs over fruit.
Bake 25-30 min or until topping is lightly browned. Serve warm with ice cream or whipped cream.