Splash a little olive oil into a saucepan and saute the onion, celery and carrot over a medium heat for 3 – 5 minutes or until the vegetables are tender.
Add the garlic and cook for another minute.
Add the tomatoes, water, oregano and salt. Stir to combine, increase the heat and bring to the boil. Cover, reduce heat and simmer for 30 minutes.
Add the basil leaves, taste and adjust seasoning if necessary. Cook for 5 minutes more. Transfer everything to a food processor and blitz to make a smooth sauce.