Heat oil in a large nonstick skillet over medium heat. Add onion, cook and stir until the onion begins to brown, about 5 minutes. Coarsely chop 1 tomato. Add it to the pan along with corn, rice, beans, chili powder, cumin, 1 teaspoon oregano and 1/2 teaspoon salt. Cook, stirring frequently, until the tomato cooks down, about 5 minutes. Let cool slightly.