Heat the oil in a large saucepan and add the tomatoes, crushing with a wooden spoon. Add the oregano and season with a little sea salt and freshly ground black pepper. Cook at a steady simmer for 20 minutes.
Meanwhile tear the mozzarella into small pieces.
Cook the spaghetti according to packet instructions until al dente then drain.
Return the spaghetti to the pan and add the parmesan, the tomato sauce and the mozzarella. Toss quickly then cover and leave for 5 minutes so that the cheese starts to melt.