Preheat oven to 375 degrees F. Coat an 8 by 8-inch baking dish (or similar 1 1/2 to 2- quart dish) with cooking spray.
Make the fruit: Combine plums, sugar, orange zest and juice in a large bowl, toss to coat. Transfer to prepared baking dish. Cover with foil and bake for 20 minutes.
Meanwhile, make the topping. Mix flour, oats, brown sugar, cinnamon and salt in a medium bowl with a fork. Add butter and blend with a pastry blender or your fingertips. Add oil and stir to coat. Add orange-juice concentrate and blend with your fingertips until dry ingredients are moistened.
When fruit has baked for 20 minutes, stir it and sprinkle topping evenly over the surface. Sprinkle with walnuts. Bake, uncovered, until fruit is bubbly and tender and topping is lightly browned, 20 to 25 minutes more. Let cool for at least 10 minutes before serving. Serve warm or at room temperature with ice cream (or frozen yogurt), if desired.