Preheat oven to 400°. Butter or grease a 9-inch cake or pie pan.
In a mixing bowl, combine oatmeal, flour, brown and white sugar, and baking soda. Cut in the butter (as you would for pie crust); with a pastry blender or with your hands, rub the butter into the dry ingredients until well blended.
Beat in one egg and vanilla extract.
Press 2/3 of the cake dough into the bottom of the pan. Spread the rhubarb jam evenly over the layer, then crumble remaining dough over top of cake.
Bake for 20-25 minutes, or until crust is golden brown.