Bring a large pot of lightly salted water to a boil. Add pasta, and cook for 8 to 10 minutes or until al dente; drain.
Preheat oven to 350 degrees F (175 degrees C).
In a skillet over medium heat, brown the ground beef until evenly brown; drain excess fat. Season with oregano, salt and pepper. Stir in tomato sauce and water. Remove from heat, and stir in cooked noodles.
In a medium bowl, combine spinach, cottage cheese, onion, and Parmesan cheese. Spread half of noodle mixture in a large casserole dish. Layer with all of spinach mixture, then cover with remaining noodle mixture.
Bake in preheated oven for 15 minutes. Cover with mozzarella, and continue cooking 10 minutes, or until cheese is melted.