1. Warm oil in a small skillet over medium-high heat. Add onion and celery and cook, stirring, until softened, 3 to 5 minutes. Add garlic and sauté 1 minute longer. Transfer mixture to a small bowl and let cool.
2. Place turkey in a large bowl. Add onion mixture, egg and oats. Use your hands to gently but thoroughly blend ingredients. Dampen hands lightly and form mixture into about 20 meatballs, each 1 1/2 inches in diameter.
3. Pour a small amount of sauce over bottom of slow cooker. Add meatballs, top with remaining sauce. Pour in 1/2 cup water. Cover and cook on high until meatballs are cooked through, 3 1/2 to 4 hours.