Heat olive oil in a large skillet over medium-high heat.
Add sliced sausage and cook, stirring frequently, until lightly browned.
Once sausage is cooked, add it to the slow cooker, along with the diced celery, onion, & carrots, minced garlic, beans, oregano, chicken broth, 2 cups of water and salt & pepper.
Mix until thoroughly combined. Cover the slow cooker and cook on low heat for 7 to 8 hours OR high heat for 3 to 4 hours.
Just before the soup is ready to serve, add fresh baby spinach and mix into the soup until wilted.