In a large bowl, combine ground turkey, breadcrumbs, grated Parmesan cheese, egg, minced garlic, chopped onion, chopped parsley, dried oregano, salt, and black pepper. Mix until all ingredients are well incorporated.
Form the mixture into 1 1/2-inch meatballs and set aside on a plate.
Heat olive oil in a large skillet over medium heat. Add the meatballs in batches, cooking for about 5 minutes per side until browned and cooked through. Remove meatballs from the skillet and set aside.
In the same skillet, pour in the marinara sauce and bring to a simmer over medium heat.
Return the meatballs to the skillet with the sauce. Cover and simmer for 10 minutes to allow flavors to meld and meatballs to heat through.
Meanwhile, bring a large pot of salted water to a boil. Cook spaghetti according to package instructions until al dente. Drain and set aside.
Divide the cooked spaghetti among four plates. Top with turkey meatballs and sauce.
Garnish each plate with chopped fresh basil and additional grated Parmesan cheese before serving.