Preheat oven to 180ºC/ 350ºF/ gas 4.
Place the rhubarb in a shallow ovenproof dish with the sugar and cinnamon.
To make the crumble topping rub the margarine into the oats, flour and cinnamon until the mixture resembles breadcrumbs. Stir in the sugar.
Spoon over rhubarb and bake for 40 minutes.