Preheat oven to 350°.
Chop rhubarb into 1 cm cubes.
In a large mixing bowl, combine chopped rhubarb and sugar until well mixed. Place into a 8" x 8" glass baking dish. The rhubarb and sugar mixture should fill up half of the baking dish.
In a separate large mixing bowl, add the brown sugar, oatmeal, flour and cubed butter. Using your hands or a pastry blender, mix together until you end up with pea sized crumbs. Place on top of the rhubarb mixture in the baking dish.
Place baking dish on a baking sheet (to capture any spill-over) and bake at 350° for 40 minutes or until topping is lightly browned and rhubarb mixture is bubbling.
When buying fresh rhubarb, look for stalks that are firm, crisp and blemish free.
Remember, the leaves of rhubarb are poisonous so only eat the stalks!
You can store rhubarb, unwashed, in the refrigerator for up to one week.