In a large heavy-based pot, drizzle in the olive oil, then add the diced onion, garlic, and oregano and sauté for 5 minutes.
Add in the fresh tomatoes, stir, and cook on low for another 20-25 minutes until the tomatoes have turned saucy.
Stir in the basil, salt, and pepper and cook for a further 5 minutes.
Use a stick blender to blend the sauce to a consistency you enjoy. Pour it into sealable jars, and place it into the fridge.