Heat oil in a Dutch oven over medium-low; add onion and cook until soft and slightly brown, about 3 minutes.
Stir in garlic cloves and cook until softened, about half a minute. Stir in crushed tomatoes OR puree; oregano, pepper, and sea salt. Increase heat and bring to a boil. Immediately reduce heat to low and simmer until thickened, about 2 hours.
Stir in fresh basil; continue cooking no more than five minutes. Add sugar to taste, if sauce is too acidic.
Remove from heat and serve with your favorite pasta.