Brown ground beef in a medium skillet over medium heat. Drain fat and put browned beef in a bowl.
Wipe out pan. Add olive oil. When hot, add onion and saute 3 minutes or until soft and translucent. Add garlic and saute 1 minute or until fragrant. Add tomato paste and saute 2-3 minutes until tomato is beginning to brown. Return the meat to the pan and add the tomato sauce, salt and oregano. Stir well and cook until hot.
In a large bowl, blend ricotta cheese with parmesan and mozzarella.
Place a layer of meat sauce in bottom of slow cooker. Top with a layer of lasagna noodles; break noodles to fit in slow cooker. Top with cheese mixture. Continue layering sauce, noodles and cheese until it's all used up.