Preheat the oven to 400°F (200°C) and line a baking sheet with parchment paper or aluminum foil.
In a mixing bowl, combine the ground beef, breadcrumbs, Parmesan cheese, parsley, egg, minced garlic, salt, black pepper, oregano, and red pepper flakes (if using). Mix well until all ingredients are fully incorporated.
Shape the mixture into small meatballs, about 1 to 1.5 inches in diameter. Place the meatballs on the prepared baking sheet.
Bake the meatballs in the preheated oven for 15-20 minutes, or until cooked through and browned on the outside.
While the meatballs are baking, heat the marinara sauce in a saucepan over medium heat until warmed through.
Once the meatballs are done, remove them from the oven and place them in the saucepan with the marinara sauce. Gently toss to coat the meatballs with the sauce.
To assemble the sliders, slice the slider buns in half and place a meatball on the bottom half of each bun. Top each meatball with shredded mozzarella cheese.
Return the assembled sliders to the oven for an additional 5 minutes, or until the cheese is melted and bubbly.
Remove from the oven, cover with the top half of the buns, and serve warm.