4 cups rhubarb, 4 cups strawberries, 1¼ cups granulated sugar, 1½ teaspoons grated orange zest, 1 tablespoon cornstarch, ½ cup orange juice, 1 cup all-purpose flour, ½ cup light brown sugar, ½ teaspoon salt, 1 cup quick-cooking (not instant) oatmeal, 12 tablespoons (1½ sticks) cold unsalted butter
