In a large Dutch oven heat the olive oil over medium heat.
Add the onion and cook for 5 minutes.
Add the garlic and cook for 30 seconds.
Add the sausage and brown it.
Add the celery, carrots, tomato and zucchini and mix.
Add the soup and dried oregano, mix and cover with a lid.
[b]If you are using a slow cooker,[/b] after cooking the onions and browning the sausage, transfer the mixture in a slow cooker, add all ingredients except spinach and cover with a lid.
Cook 2 ½ hours on [b]LOW[/b].
Remove the lid, add the spinach and cook for 30 more minutes.
Season to taste with salt and pepper, add chopped parsley, mix and serve.