Combine first 3 ingredients in a bowl; stir well. Dredge oysters in cornmeal mixture.
Coat a large nonstick skillet with cooking spray, and place over medium heat until hot; add 1 1/2 teaspoons oil. Add half of oysters, and cook 3 minutes on each side or until oysters are done and breading is golden. Repeat with remaining oil and oysters.
Cut bread in half horizontally, and spread mayonnaise and mustard evenly over cut sides of bread. Arrange lettuce and tomato slices over bottom half of loaf. Top with cooked oysters and top half of loaf. Cut loaf into 4 pieces. Serve immediately.