Heat the olive oil in a large heavy-bottomed pot over a medium-high flame. Add the onions and garlic and saute until the onions are translucent, about 10 minutes. Add the carrots and celery, season with salt and pepper and saute until all the vegetables are soft, about 10 minutes. Add the tomatoes and oregano and simmer uncovered over low heat until the sauce has thickened, about 1 hour. Season to taste with more salt and pepper.