Heat olive oil in large, heavy-bottomed pot over medium-high heat. Stir-fry green beans until skins start to blister. Add onions and cook until transluscent. Add zucchini, squash and chilis and cook for a few minutes. Add tomatoes, garlic and salt and simmer covered over low heat for 20 minutes, refraining from stirring. Add corn and oregano and cook an additional 20 minutes. Add cheddar cheese just before serving.