Cut bread lengthwise in half; place cut side up on baking sheet.
Combine oil, garlic, oregano and, if desired, pepper flakes in small bowl; mix well. Brush on cut sides of bread. Place under preheated broiler 5 inches from heat; broil 2 minutes or until bread is lightly toasted. Arrange cheese slices over bread, cutting to fit if neccessary.
Return to broiler; broil 1 minute or just until cheese is melted. Cut bread crosswise into 8 pieces.