Combine rhubarb and 2 tablespoons of flour; place in greased 8 inch square pan. Sprinkle with raisins. Combine brown sugar, cinnamon and salt; sprinkle over raisins. Combine oats and remaining flour; cut in butter until crumbly. Sprinkle over top. Bake at 375F for 40 to 45 minutes or until golden brown. Serve warm with ice cream if desired.