1. Bring a large pot of salted water to a boil. Cook pasta until al dente. Drain, lightly oil and set aside.
2. In a medium saucepan, sauté the sausage, crumbling with a wooden spoon, until no longer pink. Add onion and garlic, and continue sautéing for another 4 minutes until the sausage is cooked through.
3. Add marinara sauce to the sausage mixture and set aside. In a medium bowl, blend ricotta cheese, egg, ¼ cup of the Parmesan cheese and the chopped spinach; set aside.
4. Coat a 9-by-13-inch baking dish with olive oil, and spread 1 cup of the sauce mixture on the bottom.
5. Top with 3 lasagna noodles. Spread ¼ of the ricotta cheese mixture on the noodles and layer on 1 cup of the sauce mixture. Sprinkle ½ cup mozzarella cheese over this. Repeat this process three more times, starting with the noodles and finish with the remaining ¼ cup of Parmesan cheese. Sprinkle with oregano.
6. Preheat oven to 350° and bake for 45 minutes until hot and bubbly. Let stand 10 minutes before cutting.