Preheat the oven to 375°F/212°C. Line a baking pan with a piece of parchment paper.

Wash the stalks, cut off the ends, and discard the rhubarb leaves.

Dice the rhubarb stalks into 1/2-inch pieces and place them into a medium saucepan.

Combine the oatmeal, brown sugar, all-purpose flour, and baking soda in a large bowl. Add the cubed butter and blend using a pastry cutter. Combine all of the dry ingredients with the butter.

Press 2/3 of the crumble mixture into the bottom of the prepared pan.

Add sugar, water, and arrowroot powder to the saucepan, and stir to combine. Cook over medium heat until the rhubarb is soft and the liquid starts to thicken, about 10 minutes, while stirring frequently. Once thick, remove from the heat and stir in the vanilla extract.

Spread the rhubarb fruit filling over the pressed crumble mixture.

Sprinkle the remaining crumble mixture evenly over the top of the rhubarb layer.

Bake bars in a preheated oven for 30 minutes or until the oatmeal crumb topping is starting to turn golden brown. Remove from oven and let cool on a wire rack.