Preheat oven to 180ºC/ 350ºF/ gas 4.
Place the rhubarb in a shallow ovenproof dish and sprinkle with the sugar and cinnamon.
To make the crumble topping rub the butter into the oats and flour until the mixture resembles coarse breadcrumbs, then stir in the sugar and cinnamon and mix together well..
Spoon over the rhubarb and bake for 40 minutes.
Serve with custard, cream or ice cream.