In large bowl combine balsamic vinegar, olive oil, chopped garlic, oregano, brown sugar and mustard. Season with salt and pepper and whisk until well blended. Set aside.
Grill the sausage links for 5 minutes per side (turning once). Remove from grill. Cut each Italian link into 6 pieces. Slice onion into wedges and peppers into chunks.
Thread a piece of pepper, then sausage, then onion onto skewer; repeat, alternating pepper colors so there are a total of 4 pieces of pepper, three sausages and three onion pieces per skewer. Repeat with remaining ingredients.
Cook skewers on a grill or grill pan over medium heat about 5-6 minutes per side. Halfway through cooking process, baste skewers with balsamic sauce. Serve garnished with fresh parsley.