Preheat the oven to 450F. Drizzle the olive oil into a 9×13-inch baking dish and evenly coat the entire surface. Set aside.
Combine the ground beef, ricotta, eggs, bread crumbs, parsley, garlic, oregano, salt, and fennel in a large mixing bowl. Mix by hand until well-combined.
Roll the mixture into round, golf ball-size meatballs (about 1 ½ inches), making sure to pack the meat firmly. A medium cookie scoop works well.
Place the balls in the prepared baking dish. The meatballs should be touching one another.
Bake for 20 minutes, or until the meatballs are firm and cooked through. A meat thermometer inserted into the center of a meatball should read 160F.
While the meatballs are roasting, heat the tomato sauce in a small saucepan over mediu heat, stirring often.
When the meatballs are firm and fully cooked, remove from the oven.
Drain the excess grease from the pan.
Pour the marinara over meatballs.
Return the meatballs to the oven, and continue cooking for another 10-15 minutes.