1. Mix together flour, dry oatmeal, brown sugar, butter [or margarine] and cinnamon. Set aside.
2. In saucepan, mix together sugar, cornstarch [or flour], water, and vanilla. Cook until thickened.
3. Place half of crumb mixture in an 8 x 8 pan.
4. Spread rhubarb over crumb mixture.
5. Pour cooked mixture over rhubarb.
6. Top with remaining crumbs.
7. Bake at 350 degrees for 30-35 minutes, or until rhubarb is tender. [I do 40 minutes.]