Heat the olive oil in a large saucepan, add the onions and cook on a low heat until soft. Add the garlic and tomato purée. Cook for a few minutes then add the chopped tomatoes and oregano. Season generously and simmer for 20 mins, then allow to cool.
Store in sterilised jars in the fridge for up to 1 week, or transfer to a container and freeze for up to 2 months.