Heat the olive oil in a skillet over medium heat. Add the Italian sausage and cook until browned, breaking it into small pieces as it cooks.
Transfer the browned sausage to the slow cooker. Add the onion, carrots, celery, garlic, cannellini beans, diced tomatoes, chicken broth, and oregano.
Stir to combine, then cover and cook on low for 6 to 7 hours.
About 30 minutes before serving, stir in the chopped escarole and ditalini pasta. Season with salt and black pepper to taste.
Cover again and cook until the pasta is tender and the escarole has wilted.