Prep the pot: Lightly coat the inside of your slow cooker insert with non-stick cooking spray.
Layer the base: Lay the thinly sliced potatoes evenly across the bottom. Scatter the chopped onions directly over the potatoes.
Add the meat: Crumble the raw ground beef evenly over the onions. Sprinkle the salt, pepper, garlic powder, and oregano evenly over the meat.
Add vegetables: Layer the sliced carrots, celery, and drained green beans over the seasoned beef.
Finish the sauce: Pour the undrained diced tomatoes over the vegetables. Finally, spread the condensed tomato soup evenly over the very top.
Cook: Cover and cook on Low for 6 to 7 hours or High for 4 hours. Do not stir until right before serving, when the potatoes are completely tender.