In a medium bowl, combine the ricotta, Parmesan, 1 cup of the mozzarella, and 1/4 tsp each salt and pepper; fold in the spinach.
In a 5- to 6-qt slow cooker, combine the beef, garlic, oregano, 1/2 tsp salt and 1/4 tsp pepper. Transfer half the meat mixture to a small bowl. Add 1/2 cup of the marinara sauce to the slow cooker and mix into the meat; spread the meat evenly on the bottom of the slow cooker.
Top with a layer of the noodles, breaking them to fit as necessary. Spread 1?3 of the remaining sauce (3?4 cup) over the noodles and dollop with 1?3 of the ricotta mixture (1 cup) and half the remaining meat; repeat.
Top with a layer of noodles, the remaining sauce and remaining ricotta mixture. Sprinkle with the remaining mozzarella and cook, covered, on low for 4 hours or on high for 2 hours.