In a medium bowl, toss together carrot, red onion, vinegar, oil, ¼ teaspoon salt, and red pepper. Cover and refrigerate for at least 2 hours or up to 24 hours.
In a shallow dish, stir together cornmeal, flour, onion powder, black pepper, and remaining 1 teaspoon salt.
In a large, deep cast-iron skillet, pour oil to a depth of 2 inches, and heat over medium-high heat until a deep-fry thermometer registers 350°. Working in batches, dredge oysters in cornmeal mixture, shaking off excess. Fry until golden brown, about 2 minutes. Remove with a slotted spoon, and let drain on paper towels.
Spread mayonnaise onto cut sides of bread. Top with oysters, carrot mixture, and cilantro.