Combine oil, vinegar, parsley, oregano, garlic, salt and pepper in a small bowl. Place sausages, onion and peppers in large resealable plastic food storage bag; pour oil mixture into bag. Close bag securely, turning to coat. Marinate in refrigerator 1-2 hours.
Mix all ingredients for Horseradish-Mustard spread in a small bowl. Set aside.
Prepare grill for direct cooking.
Drain sausages, onion and bell pepper; reserve marinade.
Grill sausages, covered, 4-5 minutes. Turn sausages, and place onion and bell pepper on a grid. Brush sausages and vegetables with reserve marinade. Grill, covered, 4-5 minutes or until vegetables are crisp tender, turnign vegetables halfway through cooking time.
Move sausages and vegetables to a serving plate. Serve with Horseradish-Mustard spread.