Heat oil in a heavy saucepan over medium-high heat until shimmering. Add onion and 1 1/2 tablespoons taco seasoning. Cook until onions are soft and translucent, 3 to 4 minutes. Stir in tomatoes with juice, chicken broth, and corn.

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Sprinkle remaining taco seasoning over chicken pieces and add to soup. Stir to combine, cover, and cook until chicken is cooked through, about 5 minutes. Serve with tortilla chips, avocado slices, Oaxaca cheese, and sour cream.