Heat olive oil in a saucepan over medium-high heat. Add onion and garlic and cook 2 to 4 minutes until crisp-tender, stirring frequently.
Mix in diced tomatoes, tomato sauce, sugar, oregano and salt. Bring to a boil. Reduce heat to low and simmer 15 to 20 minutes or until flavors are blended, stirring frequently.OR.
Scald jars,pour sauce in while hot. Process for 10 minutes at 5 lbs pressure.This will make about 2-3 quarts.