In a large mixing bowl, combine oats, flour, salt, baking soda, and brown sugar. Cut in the cold, cubed butter until the mixture resembles coarse crumbs.

Set aside one-third of this crumb mixture for topping, then press the remaining two-thirds firmly into a greased 9x13-inch pan to form the base.

In a saucepan over medium-high heat, combine sliced rhubarb, sugar, cornstarch, and water. Cook while stirring constantly until the mixture thickens, about 5-10 minutes.

Remove from heat, mix in vanilla and cinnamon, and allow the filling to cool slightly before spreading it onto the crumb base.

Spread the cooled rhubarb filling evenly over the pressed base in the pan. Crumble the reserved crumb mixture on top.

Preheat your oven to 350°F (175°C). Bake the assembled bars for 30-40 minutes until the topping is golden brown and the filling is bubbling.

Allow the bars to cool completely in the pan for 1-2 hours before slicing into neat squares.