In a medium-size bowl, add olives, tomato, cheddar cheese, Monterey Jack cheese, garlic, olive oil, vinegar, oregano, salt and pepper. Mix to combine. Cover and refrigerate until ready to serve.
Heat gas grill to medium or prepare charcoal grill with medium-hot coals. Grill sausages or bratwurst for 13 to 15 minutes, turning so that all sides are grilled and sausages reach internal temperature of 160 degrees F on an instant-read thermometer.
To serve, partially slice each piece of Italian bread lengthwise to form a pocket. Place a sausage link into each. Spread each with 2 teaspoons of the mustard. Spoon about 1/3 cup of the olive and cheese mixture onto each sausage, tucking excess down into roll. Makes 6 servings.
Heat broiler. Coat broiler-pan rack with cooking spray. Place sausages or bratwurst on rack. Broil 4 inches from heat, following directions in step 2.