IN THE BOWL of a food processor fitted with a steel chopping blade, combine the peppers and garlic, and puree until smooth. Scrape the mixture into a large non-aluminum bowl. Add the chicken stock, buttermilk, sugar, salt, corn, nopales, oregano, and Tabasco, and stir to blend. Cover and chill for at least 1 hour. In a small bowl, combine the yogurt and oregano. Cover and refrigerate.
TO MAKE THE CRISPS: Turn on your broiled, and position the rack at least 5 inches from the heat. Lightly brush the top of the tortilla wedges with oil. Place them on a lightly oiled cookie sheet. Sprinkle on the oregano, crumbling it with your fingers as you do this, then drizzle on the Parmesan cheese. Run them under the broiler, just to crisp and color lightly, watching so they do not burn. Before serving, ladle the soup into bowls. Add about a tablespoon of the yogurt in the center. Sprinkle on chopped oregano and serve with the tortilla wedges.
NOTE: Although Tex-Mex and other Latin delicacies are now available in many markets across the country, pickled nopales or tender cactus leaves are less available than jalapeno peppers, for example. If you cannot find them, either chop a few pickled jalapeno peppers (with the seeds removed) or simply omit them.