Preheat large dutch oven or heavy bottom pot on low-medium heat and add olive oil. Add onion and garlic, cook for 8-10 minutes or until translucent, stirring occasionally.
Add crushed tomatoes, sugar, oregano, salt, pepper and stir. Cover with a lid and bring to a boil, then reduce to low heat and simmer marinara sauce for 30 minutes. Stir a few times to avoid scorching.
Refrigerate in an airtight container for up to 1 week. To freeze, cook and cool completely and freeze in an airtight container for up to 3 months. Thaw overnight in the fridge.