In a heavy-duty plastic bag, working in batches of 6, coat oysters with cornmeal, knocking off excess. In a heavy kettle heat 1 1/2 inches of oil to 375 deg. F. on a deep-fat thermometer and fry oysters in batches of 6, turning occasionally, until golden and just cooked through, about 1 1/2 minutes. Transfer oysters with a slotted spoon to paper towels to drain.
Halve loaves crosswise and horizontally, cutting all the way through and spread each piece with about 2 tablespoons tartar sauce. Divide tomatoes, lettuce, and oysters among bottom pieces of bread and top with remaining bread, pressing together gently.