Heat olive oil in a large skillet over medium heat. Sauté chopped onion until translucent, about 5. Stir in minced garlic and sliced mushrooms, cooking until mushrooms are tender, about 5. Add fresh spinach and cook until wilted, about 2. Remove from heat and let cool slightly.
In a mixing bowl, combine ricotta cheese and egg. Add half of the shredded mozzarella, oregano, salt, and black pepper. Mix until well combined. Fold in the cooled spinach and mushroom mixture.
Spread a thin layer of marinara sauce on the bottom of a 9x13 inch baking dish. Layer 4 lasagna noodles over the sauce. Spread half of the ricotta mixture over the noodles, followed by another layer of marinara sauce and a sprinkle of grated Parmesan cheese. Repeat layers, finishing with remaining noodles, marinara sauce, and remaining mozzarella and Parmesan cheese on top.
Cover the baking dish with aluminum foil and bake in a preheated oven at 375°F for 25. Remove foil and bake for an additional 15 until cheese is bubbly and golden brown. Let cool for about 10 before slicing.