Preheat your oven to 350°F (175°C) and gather your ingredients.

In a large bowl, mix oats, flour, baking soda, salt, and brown sugar. Cut in softened butter until crumbly.

In a saucepan, combine rhubarb, sugar, cornstarch, and water. Heat until it simmers, about 5–10 minutes.

Remove from heat and stir in vanilla extract, let cool for 10 minutes.

Press two-thirds of the crumb mixture into the bottom of a greased baking dish.

Pour cooled rhubarb filling over the pressed crumb base, spreading it evenly.

Sprinkle the reserved crumb mixture over the rhubarb filling.

Bake for 30 to 40 minutes until golden brown and bubbly.

Let cool on a wire rack for 1 hour, then slice into bars.