Heat the olive oil in a large, wide skillet or paella pan over medium heat. Add the onion, bell pepper and garlic and cook until tender, about 4-5 minutes. Add tomatoes, rice, stock, saffron, oregano, peas, salt and pepper and then bring to a boil. Reduce heat to medium low, cover and cook 10 minutes. Arrange prawns, sausage and olives on top of the rice, gently pushing them in. Cover and cook 10-15 minutes more, or until prawns are cooked and rice is tender, adding extra stock if needed. Sprinkle with parsley, top with lemon wedges, and serve.