Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.
Mix flour, brown sugar, oats, butter, and cinnamon together in a bowl until crumbly. Press 1/2 of the oat mixture into the bottom of prepared baking dish. Sprinkle rhubarb over oat mixture.
Combine white sugar, water, cornstarch, and vanilla extract in a saucepan over medium heat; cook and stir until thick and clear, about 5 minutes. Pour sugar mixture over rhubarb. Sprinkle remaining oat mixture over rhubarb.
Bake in the preheated oven until crisp is lightly brown and bubbling, about 1 hour.