In a very large pot, heat olive oil over medium-high heat. Add onion and cook until softened, about 4 minutes. Add garlic, salt, pepper, oregano, and bay leaves. Cook another minute or so, until the spices become fragrant. Add balsamic vinegar to the pot. Cook for 1-2 minutes, stirring constantly, until the vinegar is reduced in half. Add tomatoes one can at a time. Cover and simmer for 4 hours, or transfer to a slow cooker and cook on love for 4-6 hours. To freeze: Allow marinara to cool completely before transferring to freezer bags. Properly concealed in an airtight bag, this recipe will last in the freezer for up to 6 months.