Heat the oil in a large wide pot over medium-high heat. Add the onion, celery, bell pepper and garlic and cook, stirring frequently, until soft, about 5 minutes. Transfer to a large bowl and cool. Reserve the pot.
Meanwhile, add the turkey, oats, Parmesan, Worcestershire, spinach, 1/2 teaspoon salt and 1/4 teaspoon pepper to the cooled vegetables. Mix well and form into 24 small meatballs.
Pour the marinara into the reserved pot and bring to a simmer. Add the meatballs, cover and cook, turning once or twice, until cooked through, about 8 minutes.
Serve the meatballs and sauce over spaghetti and serve Parmesan on the side.
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