Preheat grill to high (about 500 degrees F).
Brush salmon red onion and sweet peppers with olive oil, salt and pepper.
For the sauce, mix parsley, oil, vinegar, garlic, oregano, crushed red pepper, salt and pepper in a medium bowl.
Oil grill rack and place the salmon, skin-side up, on the grill. Place the peppers and onions. Grill the salmon until browned, ~3 minutes each side.
Cook the peppers and onions, turning frequently, until tender and well browned, ~8 minutes.