Heat the oil in a skillet over medium-high heat and brown the sausage links on two sides, about 6 minutes total.
Transfer the links to a 6-quart slow cooker and cook the onion in the skillet until golden at the edges, 5 minutes.
Add the garlic for 30 seconds, then transfer the onion mixture to the cooker.
Pour in the tomatoes and add the oregano, salt and pepper.
Cover and cook on low for 3 1/2 hours, then stir both bell peppers into the sauce.
Cover again and cook until the sausage reaches 160°F and the peppers are tender, 1 1/2 to 2 hours more.
Toast the rolls and fill each with one sausage link, peppers, onions and drained tomato sauce.