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