Heat the olive oil in a large pot over medium heat. Add the Italian sausage and cook until browned, breaking it up with a spoon as it cooks.
Add the diced onion and cook for 4 minutes until it softens. Stir in the minced garlic and cook for 1 minute more.
Add the rinsed lentils, diced tomatoes, chicken broth, and dried oregano. Season with salt and black pepper.
Bring the mixture to a boil, then reduce the heat to low. Cover and simmer for 25 minutes until the lentils are tender.
Stir in the fresh spinach and cook for 2 minutes until it wilts. Taste and adjust seasoning if needed before serving.