Preheat oven to 350 degrees.
Combine oats, flour, brown sugar, butter and 1/4 tsp salt in medium bowl. Mix well until butter coats oats.
Press 2/3 of mixture into a 9x9-inch square pan and set aside. Reserve remaining mixture for the topping.
Combine sugar, cornstarch, water, cinnamon and pinch of salt using a wire whisk. Mix well and cook in the microwave on high for 5 minutes. Stir, and continue to cook in 2-minute intervals until the mixture thickens.
Add rhubarb to thickened cornstarch mixture, coating the rhubarb thoroughly. Place rhubarb in prepared 9x9-inch pan. Top rhubarb with reserved oatmeal mixture.
Bake 45 minutes, or until rhubarb is tender.
Serve warm with a scoop of vanilla ice cream.
Optional: You can substitute 4 cups apples or firm pears for rhubarb.