In a heavy-bottomed skillet, pour oil to a depth of 2 inches, and heat over medium heat until a deep-fry thermometer registers 350°.
In a medium bowl, stir together flour, cornmeal, salt, garlic powder, onion powder, and cayenne until combined. Dredge oysters in flour mixture, shaking off excess.
Fry until golden brown, about 3 minutes. Remove oysters using a slotted spoon, and let drain on paper towels.
In a small bowl, combine mayonnaise and Sriracha. Spread mayonnaise mixture over one side of each bread slice. Top half of bread slices with oysters, bacon, tomato, and lettuce. Cover with remaining bread slices, spread side down.