In a large pot over medium, melt butter with olive oil. Once the butter has melted, add onions and saute until they begin to soften and turn translucent; about 5 minutes. Add the garlic and cook for 1 additional minute.
Add potatoes, sausage, salt, oregano, and pepper. Stir and cook until sausage begins to brown, about 5 minutes.
Transfer the entire pot to a 5-quart or larger slow cooker and cover with 1/4 cup water.
Cover and cook on low for 6-8 hours. For the last 30 minutes of cooking, stir in the cabbage and red bell peppers.
Once the cabbage and peppers are tender, serve warm. Ideally, beside a pint of Guinness.
TO STORE: Leftovers can be refrigerated in an airtight container for up to 3 days.
TO REHEAT: Warm in the microwave for 1 to 2 minutes until heated through.
TO FREEZE: Store leftover slow cooker sausage and cabbage in the freezer for up to 3 months. Defrost overnight in the fridge before warming.