Preheat oven to 350 degrees F (175 degrees C).

Bring a large pot of salted water to a boil.

Cook lasagna noodles until al dente.

Drain the pasta, lightly oil to prevent sticking, and set aside.

In a medium saucepan, heat olive oil over medium heat.

Add Italian sausage and cook, breaking it apart with a wooden spoon, until browned.

Add onion and garlic to the sausage and continue to sauté for about 4 minutes until the sausage is fully cooked and the onions are softened.

Stir in the marinara sauce and simmer for a few minutes.

In a medium bowl, combine ricotta cheese, egg, 1/4 cup grated parmesan cheese, and chopped spinach.

Coat a 9x13 inch baking dish with olive oil.

Spread 1 cup of the sauce mixture evenly on the bottom of the dish.

Top with 3 lasagna noodles.

Spread 1/4 of the ricotta cheese mixture over the noodles.

Layer 1 cup of the sauce mixture over the ricotta cheese.

Sprinkle 1/2 cup of mozzarella cheese over the sauce.

Repeat the layering process three more times, starting with the noodles, ricotta mixture, sauce, and mozzarella.

Finish with the remaining 1/4 cup of parmesan cheese sprinkled on top.

Sprinkle the dried oregano evenly over the top of the lasagna.

Bake in the preheated oven for 45 minutes, or until the lasagna is hot and bubbly.

Let the lasagna stand for 10 minutes before cutting and serving.