In a large skillet, melt 1 tablespoon butter together with olive oil. Brown chicken in batches. Transfer to a bowl. Melt remaining butter. Add onion to skillet; cook 5 minutes, until softened. Stir in garlic; cook 1 minute more. Add stock, vinegar and oregano. Return chicken and accumulated juices to skillet. Cover and simmer 20 minutes until chicken is cooked through. Sprinkle with parsley before serving.