Preheat oven to 375° F. Coat an 8-by-8-inch baking dish (or similar 1 1/2- to 2-quart dish) with cooking spray.
To prepare filling: Combine plums, sugar, orange zest, orange juice in a large bowl; toss to coat. Place filling in the prepared baking dish. Cover with foil and bake for 20 minutes.
Meanwhile, make 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 the filling has baked for 20 minutes, stir it and sprinkle topping evenly over the surface. Sprinkle with almonds (or walnuts). Bake, uncovered, until the fillin 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.