In a mixing bowl, combine the ground beef, breadcrumbs, egg, grated Parmesan, minced garlic, oregano, salt, and black pepper. Mix gently until the ingredients are evenly distributed.
Shape the mixture into 12 to 16 meatballs, each about 1.5 inches wide. Set them aside on a plate.
Heat the olive oil in a large skillet over medium heat. Add the meatballs and cook for 8 to 10 minutes, turning occasionally until browned on all sides.
Pour the marinara sauce into the skillet. Reduce the heat to low, cover, and simmer for 15 minutes until the meatballs are cooked through and the sauce has thickened slightly.
While the meatballs simmer, bring the water to a boil in a medium saucepan. Slowly whisk in the polenta. Reduce the heat to low and cook for 5 to 7 minutes, stirring often, until the polenta thickens and becomes creamy.
Spoon the polenta onto plates. Top each serving with meatballs and sauce. Finish with fresh basil leaves and additional grated Parmesan.