Preheat oven to 375°F (190°C).
In a large saucepan, heat olive oil over medium heat. Add chopped onion and minced garlic, cook until softened.
Add ground beef, cook until browned. Stir in marinara sauce, salt, pepper, and oregano. Simmer for 10 minutes.
Cook lasagna noodles according to package instructions. Drain and set aside.
In a mixing bowl, combine ricotta cheese, 1 cup mozzarella cheese, and ¼ cup Parmesan cheese.
In a baking dish, spread a thin layer of meat sauce. Layer with noodles, ricotta mixture, and meat sauce. Repeat layers, ending with meat sauce. Top with remaining mozzarella and Parmesan cheese.
Cover with foil and bake for 25 minutes. Remove foil and bake for an additional 25 minutes, until cheese is golden and bubbly.
Let lasagna rest for 10 minutes before serving.