Preheat your oven to 400°F.
In a large mixing bowl, combine the ground beef, breadcrumbs, Parmesan cheese, parsley, onion, garlic, egg, salt, black pepper, oregano, and red pepper flakes. Mix until just combined; do not overmix.
Using your hands, shape the mixture into meatballs about 1 to 1.5 inches in diameter and place them on a baking sheet lined with parchment paper.
Bake for 20-25 minutes or until cooked through and browned.
While the meatballs are baking, heat the marinara sauce in a large skillet over medium heat.
Once the meatballs are done, transfer them to the skillet and simmer in the sauce for an additional 5-10 minutes.
Serve the meatballs with pasta, on a sub roll, or on their own with extra sauce.