Preheat oven to 350°F (175°C). Grease or line an 8x8-inch baking dish.
In a large bowl, combine oats, flour, brown sugar, baking soda, and salt.
Add melted butter and mix until crumbly. Reserve ½ cup for topping.
Press remaining mixture into the bottom of the prepared pan.
In a saucepan, combine rhubarb, granulated sugar, cornstarch, lemon juice, and vanilla. Cook over medium heat until thick and jammy (5–7 minutes).
Spread rhubarb mixture over crust. Crumble reserved oat mixture on top.
Bake for 25–30 minutes or until golden and bubbly.
Cool completely before slicing into bars.