In a small bowl, whisk together oregano, cumin, onion, garlic, lime juice, orange juice, and beer until combined.

Season pork chunks with salt and pepper.

Set Instant Pot to Sauté. Add olive oil. Once hot, add half the pork and sear, turning every 45 to 60 seconds. Pork should be browned on all sides. Transfer pork to a separate clean plate and repeat with remaining pork until browned. Press Cancel to turn off the heat.

Pour sauce on top of the pork. Close lid and set to Manual pressure on High. Cook for 30 minutes. Let naturally release for about 15 minutes before removing lid.

Set oven broiler to high heat.

Remove pork to a large cutting board and shred with two forks. Spread pork onto a large baking sheet. Spoon one third of the juices on top of the pork, tossing to combine. Broil for 4 to 5 minutes, until the pork edges begin to crisp.

Remove the baking sheet from the oven and top with half of the remaining juices in the Instant Pot. Toss again to combine. Broil for an additional 5 minutes.

Remove the baking sheet from the oven and top with remaining juices. Toss to combine.

To assemble the tostadas, top tortillas with even amounts of crispy pork. Top with sliced red onion, diced avocado, jalapeno, cilantro, and cotija cheese. Serve with lime wedges.