Blanch tomatoes: place tomatoes whole in a boiling water for about a minute before removing them. This will allow the tomato skin to be easily removed. Peel and cut tomatoes into segments, remove the seeds and heat in a pan with a tablespoon of oil and a pinch of salt.
Heat the oil in a saucepan. Brown the crushed garlic together with a whole chilli. Remove the garlic from the sauce pan, set aside, and add the tomatoes. Slowly cook on low heat for about 10 minutes. Adjust the salt and sprinkle with pepper.
Cook the spaghetti in plenty of salted water, drain and toss with the sauce. Add the diced mozzarella and mix well. Sprinkle with oregano and serve.