In a large saucepan, heat the olive oil over medium heat. Add the minced garlic and diced onion, and sauté until the onion is translucent and the garlic is fragrant, about 3-4 minutes.
Pour in the whole San Marzano tomatoes with their juice. Using a wooden spoon, break the tomatoes into smaller pieces. Stir in the salt, black pepper, and dried oregano.
Bring the sauce to a simmer and reduce the heat to low. Allow the sauce to cook, uncovered, for about 1 hour, stirring occasionally to prevent sticking.
Remove the sauce from heat and stir in the chopped fresh basil. For a smoother sauce, use an immersion blender to puree the sauce to your desired consistency.