Heat the olive oil over medium heat. Add the onion, carrots, celery, potato and garlic. Cook for 5 minutes until vegetables are soft.Add the oregano, salt and pepper and stir.Pour the vegetable broth and water and bring to a boil.Stir in the squash, rice and bay leaf. Reduce heat to a low simmer and cook for 20 minutes until rice is cooked through.Reduce heat to low and stir in lemon juice.