Preheat oven to 400°F (200°C).
Line a baking sheet with foil or parchment paper.
Place sausages, sliced peppers, and onions on the sheet.
Drizzle with olive oil and sprinkle with garlic powder, oregano, salt, and pepper.
Toss everything together on the pan.
Bake for 25–30 minutes, flipping sausages halfway through, until sausages are cooked through and vegetables are tender.
Optional: Broil for 2–3 minutes at the end for extra color.
Serve as-is, or with rice, pasta, or in a hoagie roll.