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. 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 with black pepper and stir in the ketchup.
Freeze in batches in small containers for up to 1 month.
Cook's tip: it's best to keep the tomatoes whole while the sauce cooks, as the pips can release a bitter taste. Wait until the end before breaking them up.