Brown sausage in oil; transfer to slow cooker.
Sauté onions, garlic, tomato paste, oregano, and red pepper flakes; cook until softened.
Deglaze with wine; simmer and add to slow cooker.
Stir in crushed, diced, and pureed tomatoes. Cook 8–10 hours (low) or 5–7 hours (high).
Microwave peppers until tender.
Stir peppers into sauce; season with salt, pepper, and sugar.
Add parsley before serving.