Juice 4 limes until you have about 1/2 cup juice. Juice 1 orange (about 2/3 cup juice). Smash 4 garlic cloves. Trim off any large pieces of fat from the surface of a 3 1/2 to 4-pound boneless pork shoulder. Cut the pork into 3 pieces. 

Season the pork with 1 tablespoon kosher salt, 2 teaspoons dried oregano, 1 teaspoon ground cumin, and 1/2 teaspoon black pepper.

Heat an Instant Pot or electric pressure cooker on the Sauté setting. Add 1 tablespoon vegetable oil. Add 1 piece of pork and sear on all sides, 2 to 3 minutes per side. If the pressure cooker registers a burn warning, remove the pork from the pot, pour in 1/4 cup water, and scrape up any burned bits. Return the pork to the pot and continue cooking until browned. Transfer the pork to a plate, and repeat with the remaining pieces of pork. 

Turn off the pressure cooker. Add the lime and orange juices and use a wooden spoon to scrape up the browned bits from the bottom of the pot. Return the pork to the pot in an even layer. Add the garlic.

Lock the lid on and make sure the pressure valve is set to seal. Cook on HIGH pressure for 55 minutes. It will take 7 to 10 minutes to come up to pressure.

Once the cook time is done, let the pressure release naturally for 15 minutes. Meanwhile, arrange a rack in the top third of the oven and heat the broiler to high.

Quick release any remaining pressure. Transfer the pork to a rimmed baking sheet, and use 2 forks to shred the meat into bite-sized pieces. Discard any large pieces of fat. Spread the carnitas into an even layer. Broil the carnitas until the ends are browned and crisp, 2 to 3 minutes.

Spoon some of the cooking liquid over the carnitas and serve on warm tortillas with desired toppings.