Preheat oven to 350°F (175°C).
In a large bowl, mix together the rhubarb, strawberries, and 1 cup of granulated sugar.
Transfer the fruit mixture to a deep-dish pie dish or a baking dish.
To make the topping, combine the flour, oatmeal, granulated sugar, brown sugar, ground cinnamon, and salt in a separate bowl. Once well mixed, stir in the melted butter until the mixture becomes crumbly.
Evenly sprinkle the topping over the fruit mixture.
Place the baking dish in the preheated oven and bake for 40-50 minutes, or until the filling is bubbly and thickened. If the mixture is still too liquid, bake for an additional 10-15 minutes. The topping should be golden brown with some darker areas.