Cut the flank steak into thin strips about 1/4" wide (against the grain).
In a mixing bowl or gallon-sized resealable bag, mix together olive oil, orange juice, lime juice, soy sauce, white vinegar, chili powder, paprika, cumin, garlic powder, onion powder, oregano, salt, and ground black pepper and mix together to make a marinade.
Add the pieces of steak to the bag, cover bowl (or seal bag closed) and let steak marinade for at least 30 minutes in the fridge (but you can go as long as overnight if you want!).
Pour steak and marinade into the Instant Pot.
Cover steak with diced onion.
Place lid on top of Instant Pot and move valve to SEALING.
Press PRESSURE COOK (or MANUAL) button and cook for 7 minutes.
When timer is finished, allow pot to do a NATURAL RELEASE (should take about 15-20 minutes).
Remove lid, stir meat, and serve on tortillas with your favorite toppings. The meat should fall apart - mine practically shredded on it's own because it was so tender.