Preheat the oven to 350°F (175°C). Lightly grease a 9x13-inch baking dish.
In a large bowl, toss together the rhubarb, apples, granulated sugar, cornstarch, vanilla extract, cinnamon, and a pinch of salt until evenly coated.
Spread the fruit mixture evenly in the prepared baking dish.
In a medium bowl, stir together the oats, flour, brown sugar, cinnamon, and salt. Add the melted butter and stir until crumbly.
Sprinkle the oat mixture evenly over the fruit filling.
Bake for 40–45 minutes, until the topping is golden brown and the filling is bubbling.
Let the crisp cool for about 10–15 minutes before serving. Enjoy warm with ice cream or whipped cream if desired!