Preheat your oven to 350°F (175°C).
Prepare a baking dish by greasing it lightly.
In a bowl, combine the chopped rhubarb, granulated sugar, cornstarch, and lemon juice. Mix well.
Spread the rhubarb mixture evenly in the greased baking dish.
In another bowl, mix the rolled oats, flour, brown sugar, cinnamon, and salt.
Pour the melted butter into the oat mixture and stir until crumbly.
Evenly distribute the oat topping over the rhubarb layer.
Bake in the preheated oven for 35-40 minutes or until the topping is golden brown.
Remove from the oven and allow to cool slightly before serving.
Serve warm with ice cream or whipped cream, if desired.