In a medium bowl, combine the oats, flour, brown sugar, salt, and baking soda and stir to combine. Add the butter and use clean hands to mix the butter into the dry mixture. Mix, squeezing the butter into the oat mixture, until it is well blended and the texture is like cookie dough. Set aside.
For the Filling and Baking
In a large bowl toss together the rhubarb, lemon juice, vanilla, sugar and tapioca. Transfer to an 8-inch-square baking dish or pan, or a 1½ qt baking dish. Spread rhubarb into an even layer in the dish.
Crumble oat topping over the rhubarb in an even layer. Place dish on a foil-lined baking sheet and transfer to the oven.
Bake until juices are bubbling throughout, and crisp-topping is deep golden brown, 38 to 45 minutes. Remove from the oven and let cool 15 to 30 minutes. Serve warm with ice cream.
• Nutrition facts are without ice cream.• I cut the rhubarb into ½-inch pieces.• You can add nuts to the toppings: Reduce the oats by ¼ cup and add ½ cup chopped pecans.