Combine olive oil, lemon juice, oregano and salt and pepper in a medium bowl, whisk until well combined. Remove 1 tablespoon to a small bowl. Add the prawns to the medium bowl and toss gently to coat.
Preheat a greased barbecue plate on medium-high heat. Add the prawns and marinade to the hot barbecue, and cook for 2-3 minutes, turning every minute or until the prawns are cooked through. Remove to a plate, cover to keep warm. Increase the heat to high, add mushrooms and cook for 2-3 minutes each side until just tender.
Transfer the mushrooms, stalk-side up to serving plates. Combine the hummos and yoghurt, then spoon over the mushrooms. Top with rocket and prawns. Spoon over the reserved dressing, season with salt and pepper and serve with barbecued flat bread.