1-Prepare the crumb base: Combine oats, flour, salt, baking soda, and brown sugar; cut in cold butter until coarse crumbs form, then set aside 1/3 for topping.

2-Press the base: Grease a 9×13 inch pan and firmly press the remaining 2/3 of the crumb mixture into the bottom.

3-Make the rhubarb filling: Cook chopped rhubarb, sugar, cornstarch, and water over medium-high heat, stirring frequently, until thickened and glossy (5–10 minutes); remove from heat and stir in vanilla and cinnamon, then let cool slightly.

4-Assemble the bars: Spread the rhubarb filling evenly over the crust, then sprinkle the reserved crumb mixture on top.

5-Bake: Preheat oven to 350°F (175°C) and bake for 30–40 minutes, rotating the pan halfway through, until the topping is golden brown and the filling bubbly.

6-Cool and slice: Let the bars cool completely in the pan for 1–2 hours before slicing into neat squares.