In a large mixing bowl, combine oats, flour, salt, baking soda, and brown sugar. Cut in cold, cubed butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs, about 5 minutes. Set aside one-third of the mixture for the topping. Press the remaining mixture firmly into the bottom of a greased 9x13-inch pan to form a uniform base.

In a medium saucepan, cook the chopped rhubarb with sugar, cornstarch, and water over medium-high heat. Stir frequently and continue cooking for 5 to 10 minutes until the rhubarb breaks down and the mixture thickens and becomes glossy. Remove from heat and stir in vanilla and ground cinnamon.

Spread the prepared rhubarb filling evenly over the pressed crumb base in the pan. Crumble the reserved topping mixture over the rhubarb layer.

Preheat your oven to 350°F (175°C). Place the assembled bars in the oven and bake for 30 to 40 minutes, or until the topping is golden brown.

Once baked, remove the pan from the oven and let the bars cool in the pan for 1 to 2 hours. Then slice into squares or rectangles.