Puree tomatoes and garlic in your blender until smooth.
Heat olive oil in a saucepan. Add chilli flakes, oregano and onion. Cook for a few minutes until the onion is softened. Add the tomato puree and salt. Simmer on medium low for about 20-25 minutes, till the sauce is thickened and chunky.
Taste and adjust for salt. You may need to add a bit of sugar if the sauce is tart.
You can also puree the sauce to make it very smooth. Store in an airtight container in the refrigerator.