Cut the onion into small cubes, chop the garlic very finely.
Heat the olive oil in a medium pot over medium heat, add the onion and cook for about 3-4 minutes, until soft and translucent.
Add the garlic and oregano (and dried basil if you're not using fresh basil leaves), cook for 30 seconds or until fragrant.
Add the tomatoes, bring to a boil, partially cover with a lid, and simmer (cook over low heat) for 15 minutes.
Add finely chopped basil, blend the sauce with a hand/immersion blender (I'm blending it only partially, not until completely smooth), and season with salt and pepper to taste. If you don't want to blend your sauce, finely chop basil and add to the sauce.