In a large bowl, combine ground beef, breadcrumbs, 1/4 cup grated Parmesan cheese, 1 large egg, 2 cloves minced garlic, chopped parsley, dried oregano, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Mix gently until just combined.
Shape the mixture into 1 1/2-inch meatballs and place them on a baking sheet.
Heat olive oil in a large skillet over medium heat. Add the meatballs in batches and brown on all sides, about 5 minutes per batch. Remove browned meatballs and set aside.
In the same skillet, add chopped onion and cook until softened, about 5 minutes. Add 3 cloves minced garlic and cook for 1 minute until fragrant.
Stir in crushed tomatoes, tomato sauce, dried basil, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Bring to a simmer.
Return the browned meatballs to the skillet with the sauce. Cover and simmer on low heat for 25 minutes, stirring occasionally.
While the meatballs simmer, bring a large pot of salted water to a boil. Cook spaghetti according to package instructions until al dente. Drain and set aside.
To serve immediately, toss cooked spaghetti with some of the sauce and meatballs. Sprinkle with grated Parmesan cheese and chopped fresh basil.
To make ahead, cool the meatballs and sauce completely, then store in an airtight container in the refrigerator for up to 3 days or freeze for up to 3 months.
Reheat gently on the stovetop over low heat until warmed through. Reheat cooked pasta separately by briefly immersing in boiling water or microwaving with a splash of water.