In screw-top jar combine olive oil, vinegar, oregano, mustard, and crushed red pepper. Cover; shake well. Set aside.
On six of the bread slices layer tomatoes, cheese, prosciutto, basil, and pepperoncini salad peppers. Shake oil mixture. Drizzle oil mixture over top of peppers.
Shake oil mixture. Drizzle oil mixture over peppers. Top with remaining six bread slices. Wrap tightly. Place sandwich on a cutting board or tray. Place a shallow baking pan on top of sandwich. Chill up to 2 hours before serving. Makes 6 main-dish sandwiches.