Heat oil in a large nonstick skillet over medium heat. Add onion and corn; cook, stirring, until the onion begins to brown, about 5 minutes. Coarsely chop one tomato. Add it to the pan along with rice, beans, chili powder, 1 tsp oregano and salt. Cook, stirring frequently, until the tomato cooks down, about 5 minutes. Let cool slightly.
Coarsely chop the remaining tomatoes. Combine with cilantro, salsa and the remaining oregano in a medium bowl.Toss lettuce in a large bowl with the bean mixture, half the fresh salsa and 2/3 cup cheese. Coarsely crumble the tortilla chips. Serve salad sprinkled with chips and the remaining cheese, passing lime wedges and the remaining fresh salsa at the table.