In a pot heat the olive oil and add the onions.
Saute for a minute or two and add garlic.
Saute for a minute and add the carrots and celery with some salt and pepper.
Add the crushed tomatoes.Add the oregano,salt,pepper and mix.
Bring to a boil, cover and cook on low for about 15 minutes until all incorporated.Add the reserved liquid to the sauce to thicken.
Your marinara sauce is ready.