Heat olive oil in saucepan over medium until shimmering. Add minced garlic, oregano, and red pepper flakes. Cook for 30 seconds.
Add tomatoes and onion powder. Salt to taste. Continue to cook, simmering until reduced to desired consistency. A thinner sauce will be ready after 5-10 minutes of simmering; for a thicker sauce, the full 30 minutes.
Note: If you used peeled whole tomatoes, transfer the sauce to a blender or food processor, or use an immersion blender right in the sauce pan, and blend until smooth (or desired consistency). Crushed tomatoes do not need to be blended.