Place ground chicken, egg, Parmesan cheese, breadcrumbs, garlic, oregano and 1/2 teaspoon salt in large bowl. Knead mixture together with hands until evenly combined. Divide mixture in half; break apart and shape into 1-inch balls.

In large, non-stick skillet, warm olive oil over medium-high heat. Place meatballs in pan, up to eight at a time, and cook until all sides are browned, approximately 5 minutes for each meatball. Remove meatballs from pan with slotted spoon; place on plate.

Reduce heat on stove to medium-low. Add onion; saut until tender, about 3 minutes. Stir in tomato sauce and diced tomatoes. Simmer mixture for 10 minutes. Gently add all meatballs to sauce, cover pan and simmer 20 minutes. Sprinkle 1/2 teaspoon of salt and freshly ground black pepper into mixture.

While sauce is cooking, cook spaghetti according to package directions.

Divide cooked spaghetti among 4 large, shallow bowls or plates. Top with meatballs and sauce. Pass 1 cup of Parmesan cheese separately for topping.