In a large bowl, combine rhubarb, apple, sugar, corn starch and cinnamon. Set aside. In another bowl, mix oats, salt, flour, cinnamon and brown sugar; mix in melted butter, ensuring everything is well combined.
Spray the slow cooker with a non-stick baking spray. Place apple and rhubarb mixture on the bottom, top with oat crumble. Cook on low for 2½ hours; remove the lid and cook for another hour. Allow to sit for 15 minutes before serving. Serve with vanilla ice cream. Makes 4-6 servings