Strawberry Rhubarb Crisp

Name: Strawberry Rhubarb Crisp
Ingredients: 4 cups fresh rhubarb, 1-inch diced (4 to 5 stalks); 4 cups fresh strawberries, hulled and halved, if large; 1 1/4 cups granulated sugar, divided; 1 1/2 teaspoons grated orange zest; 1 tablespoon cornstarch (see note); 1/2 cup freshly squeezed orange juice; 1 cup all-purpose flour; 1/2 cup light brown sugar, lightly packed; 1/2 teaspoon kosher salt; 1 cup quick-cooking (not instant) oatmeal, such as McCann's; 12 tablespoons (1 1/2 sticks) cold unsalted butter, diced; Vanilla ice cream, for serving