Brown ground beef in pan over medium-high heat. Add tomato sauce (reserving a tablespoon) and oregano. Simmer until heated through. Remove from heat.
In a bowl, mix spinach, ricotta cheese and 1/8 cup Parmesan cheese together. Set aside.
Fresh lasagna noodles are preferred, but you can use oven-ready noodles too. In a 9x13 pan, spray lightly with vegetable oil. Add the remaining tomato sauce (mixed with a little water) to the 9x13 pan.
Start the first layer of lasagna noodles, topped by a sprinkling of mozzarella cheese. Follow with the meat mixture, then lasagna noodles. Another sprinkling of mozzarella cheese, topped by the spinach mixture and lasagna noodles. Finish the top layer with the remaining cheeses.
Bake covered at 375 F for 45 minutes, then uncover and broil until golden on top (about 10 to 15 minutes).
Let stand for 5 minutes before cutting and serving. Enjoy!