In a large mixing bowl whisk together the prepared dressing, reserved mushroom juice, dry oregano and red pepper flakes. Set aside.
Cook the pasta in salted water per the package directions until al dente. Drain well.
Add the cooked pasta and remaining ingredients to the bowl and toss until combined.
Adjust the salt and black pepper to your taste.
Refrigerate for at least 4 hours preferably overnight before serving.
Stir well before serving.