Heat half the oil in a frying pan, add the mushrooms and sauté for 3-4 mins or until golden. Transfer to a plate and set aside.
Beat the eggs together with the oregano, salt and freshly ground black pepper.
Heat the remaining oil in the frying pan, add the eggs and cook over a medium heat for 2-3 minutes During this time, use a spatula to push the cooked egg into the centre of the omelette, then tip the pan so that the raw egg fills the gap and repeat until no runny egg remains and the base is golden.
Place the omelette under a hot broiler and cook for 1-2 mins or until the top has set. Spoon over the tomato sauce and top with the cheese and mushrooms. Season with more black pepper and a little salt, to taste.
Pop back under the boiler and cook for 2 mins or until the cheese is pale golden.