Heat the oil in a large, heavy-based pan over a medium heat. Add the onion, garlic and oregano, and cook gently, stirring occasionally, for about 8 minutes until the onion has softened.
Gently tip the tomatoes into the pan and stir into the onions, trying not to break up the tomatoes. Cook over a low heat for 30 minutes, stirring occasionally.
Break up the tomatoes in the pan – you can use a pair of kitchen scissors to snip them. Season and stir in the ketchup. Freeze batches of the sauce in small containers and simply defrost them to use in a variety of dishes. Use up within a month.