Into a medium saucepan over medium heat, add butter. When butter is melted, add onions. Sauté 2 minutes, until onions are softened.
Add salt and oregano. Continue cooking 2 minutes more, stirring frequently, until onions are just beginning to turn golden in color.
Add garlic and cook for an additional 1 minute.
Pour in crushed tomatoes and add sugar. Increase heat to medium high for 1-2 minutes, just enough to heat through. Reduce heat back down to low and simmer 10 minutes.
Remove from heat then stir in olive oil and basil.
Use salt, pepper, and red pepper flakes to adjust seasoning to taste.