1. Preheat the oven to 300°F.
2. Toss the mushrooms in a bowl with 2 tablespoons olive oil and place them, stem side down, on a baking sheet. Roast until very tender and well browned, stirring halfway through, about 35 minutes. Cool completely. Slice the mushrooms and combine them with the juice from the roasting pan. Set aside.
3. To make the salad, toss the tomatoes, ¼ cup olive oil, the basil, orange juice, oregano, parsley, salt, and pepper in a bowl. Set aside. (The salad can be prepared to this point up to 1 day in advance. Refrigerate the mushroom mixture and tomato mixture separately and bring them to room temperature before continuing.)
4. Wash the barley well under cold running water. Stir the barley into a large saucepan of boiling water, reduce the heat, and simmer until tender, about 20 minutes. Drain well and rinse under cold running water just until tepid. Drain well. Transfer the barley to a serving bowl large enough to hold all the ingredients. Add the tomato mixture and the mushrooms and their liquid. Toss well. Let stand for 30 minutes to 1 hour, tossing occasionally, before serving.