Preheat the oven to 400°F (200°C).
Line a large rimmed baking sheet with parchment paper or aluminum foil for easy cleanup.
Place the sliced bell peppers and onions onto the prepared baking sheet.
Drizzle the vegetables with olive oil and sprinkle with minced garlic, dried oregano, salt, black pepper, and red pepper flakes.
Toss the vegetables until evenly coated and spread them in a single layer.
Nestle the whole sausage links among the vegetables on the sheet pan.
Roast in the center of the oven for 25 to 30 minutes, turning the sausages and tossing the vegetables halfway through, until sausages reach an internal temperature of 160°F (71°C) and vegetables are tender and slightly charred.
Optional: Switch the oven to broil for the last 2-3 minutes to achieve additional browning.