Combine the chicken breasts and chicken stock in a pot and bring to a simmer over moderate heat. Simmer covered 20 minutes. Remove the chicken, let cool, discard the skin and bones, and shred the meat.
Meanwhile, heat the oil in a skillet over moderate heat. Saute the onion, peppers, and garlic until tender, about 10 minutes.
Add the shredded chicken, the onion mixture, and the remaining ingredients (corn, tomatoes, cumin, chili powder, oregano, salt, and pepper) to the pot. Simmer 20 to 30 minutes.
Garnish with toasted tortilla strips, scallions, and cheese or sour cream if desired. Serves 4 to 6.