Mix together the rhubarb, sugar, flour and cinnamon in a medium bowl before placing in an 8x8 glass baking dish.
Mix together the butter, oats, flour, brown sugar, salt and cinnamon in a bowl (I use the same bowl so that I don't dirty a second one). Sprinkle topping over rhubarb mixture.
Bake for 30 minutes – 35 minutes, or until topping starts to brown.