For the tomato sauce, heat the olive oil over medium-high heat in a heavy-bottomed saucepan. Add the onion and sauté for 5 minutes, being sure to not let the onions brown (turn down heat if necessary). Add garlic and sauté another minute then stir in the tomatoes, salt, pepper, oregano and bay leaf. Bring sauce to a boil then turn down to a simmer and cook 1 1/2 hours, stirring occasionally and adjusting heat if necessary. You can do this up to several days before you make the green beans. Discard bay leaf.
To cook the green beans, bring the sauce up to a boil over high heat in a large saucepan or stock pot, add the green beans and cover, cooking 8 to 10 minutes, stirring occasionally to keep sauce from sticking to the bottom of the pot. If you need to fix this for less than eight people, make all the tomato sauce and store in individual containers then cook with green beans, as needed.