Bring a large pot of salted water to a boil. Cook pasta until al dente. Drain, lightly oil and set aside.
In a medium sauce pan, 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.
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.
Coat a 9x13” baking dish with olive oil and spread 1 cup of the sauce mixture on the bottom.
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.
Preheat oven to 350 degrees F and bake for 45 minutes until hot and bubbly. Let stand 10 minutes before cutting.