Preheat the oven to 350 Degrees F. In a 10x6x2 inch baking dish, combine the first 6 ingredients. Cream the butter with the four and brown sugar, then stir in the oats. Sprinkle over the rhubarb.
Bake in a moderate oven, 350 Degrees F. for 40 minutes or until the rhubarb is tender and the top is brown.