Put diced rhubarb in a 2 Quart baking dish or a deep cast iron skillet and pour sugar over, stir to combine.
For the topping, mix flour, baking powder, sugar and salt. Cut in butter until the mixture is crumbly, then add oats and combine everything together, add milk and stir until just moistened.
Drop spoonfuls on the rhubarb mixture leaving spaces here and there. Bake uncovered for 50 minutes until the rhubarb is cooked and the top is golden and crusty.