In a large skillet, heat the oil over medium heat. When it's hot, add the garlic and onion and saute for 2 minutes.
Add the tomatoes (with the liquid they came in), oregano, salt, pepper and optional red pepper flakes if using.
Bring the liquid to a simmer over medium-high heat, then reduce to a low simmer and cook for about 15-20 minutes. Using a spatula, gently break apart the whole tomatoes as they cook. They'll continue to break down a little as they simmer, so leave them a little larger than you want them to be in the final sauce.
During the last five minutes, add four whole fresh basil leaves if desired. Remove the pan from heat and scoop out the leaves before serving. Add more salt and pepper to taste if needed. If desired, add a light dusting of lemon zest to brighten up the flavor. You can also top with chopped fresh basil or parmesan cheese if desired.