In a large bowl, combine meat, egg, breadcrumbs, Parmesan cheese, garlic, parsley flakes, oregano, salt, and pepper. Use your hands or a fork to gently mix. Shape the meat mixture into 2-inch balls.
Heat oil in a large skillet over medium heat until oil shimmers. Add the meatballs to the skillet (working in batches, if necessary) and brown on all sides, about 5-7 minutes. Add more oil to the skillet, as needed, to prevent the meatballs from sticking. Remove meatballs from the skillet and drain on paper towels.
Warm the marinara sauce in a large pot or Dutch oven. Add the meatballs and bring to a boil. Reduce the heat to low and simmer, uncovered, for 20-30 minutes (or until the meatballs are cooked through and no longer pink). If the sauce starts to get too thick as it simmers, you can cover the pan at any point.