Bring a large pot of salted water to a boil. Combine ground chicken, breadcrumbs, Parmesan, egg, 3 minced garlic cloves, parsley, ½ tsp salt, and pepper. Mix gently until just combined.

Roll into 1½-inch meatballs. Heat 2 tbsp oil in a skillet over medium-high. Sear meatballs, turning, until browned, about 8 minutes. Transfer to a plate.

Reduce heat to medium, add remaining 1 tbsp oil and onion. Cook until soft, 4-5 minutes. Stir in remaining garlic, cook 30 seconds. Add crushed tomatoes, oregano, ½ tsp salt, and red pepper flakes. Bring to a simmer.

Return meatballs to sauce, cover, and simmer on low 10-12 minutes. Cook spaghetti until al dente; drain, reserving ½ cup pasta water.

Add spaghetti to the skillet, toss with sauce and meatballs, adding pasta water if needed. Top with fresh basil and serve.