Season the pork butt with salt and pepper.
Heat oil in a cast iron skillet.
When oil is hot brown the pork for a few minutes on each side. Do this in batches so you don't crowd the skillet.
Preheat oven to 275.
Transfer browned pork to a casserole dish.
Top with the peeled garlic, onion slices, oregano and finally the coca-cola.
Cover the casserole dish tightly with foil and bake in oven for 3 hours.
Take out the casserole dish and remove the foil. Shred the pork pieces with two forks. Removing any fat. Also remove the garlic and onions.
Turn on the broiler and return casserole dish to oven.
Let broil for a few minutes then take out and stir and return to broiler for another few minutes.
Repeat this process until you get your desired level of crispiness. Most of the coca-cola should be evaporated by now.
Season with more salt and pepper.
Serve with warm corn tortillas.