Heat olive oil in a large pot or Dutch oven over medium heat.
Add chopped onion and cook until softened, about 5-7 minutes.
Add minced garlic and cook for another minute until fragrant.
Pour in the crushed tomatoes and water. Stir to combine.
Add dried oregano, dried basil, red pepper flakes (if using), salt, and black pepper. Stir well.
Bring the sauce to a simmer, then reduce heat to low, cover, and let it cook for at least 30 minutes, stirring occasionally. The longer it simmers, the deeper the flavor.
Taste and adjust seasonings as needed.
Serve hot over your favorite pasta.
Notes
This sauce can be made ahead of time and stored in the refrigerator for up to 5 days or frozen for longer storage.