Place whole peppers over a high flame, rotating frequently to char the skin, for approximately 3 to 6 minutes. Once a dark char has developed on the peppers, wrap in aluminum foil to steam. After 5 to 10 minutes, remove from the foil and scrape skins off of the peppers. Slice open to de-seed. Spread charred peppers, shredded Oaxaca cheese and sliced avocado evenly between the 8 tortillas. Wrap each taco in individual foil square and crimp to close. Place foil packets on hot grill or coals for 3 minutes then turn and finish for an additional 3 minutes.