Combine the beans, water, and salt in the Instant Pot. Leave the pot turned off and let the beans soak for 10 to 12 hours or up to overnight.
After soaking, secure the lid and select Pressure Cook on high for 15minutes.
When the cooking program has finished, allow the pressure to release naturally for 15 minutes, then quick release any remaining pressure.
Press cancel and open the pot, then ladle out 250ml of the cooking liquid, and set the liquid aside. Then carefully drain the remaining liquid form the inner pot, and leave the beans to one side.
Return the now-empty inner pot to the Instant Pot and select Sauté, setting the time for 5 minutes.
Add the oil, garlic, onion, and celery and sauté until the onion has softened. Add the drained beans, the reserved liquid, chopped tomatoes, oregano, and pepper and stir well.
Use a wooden spoon to deglaze the base of the inner pot, then secure the lid.
Press the Cancel button to reset the cooking program, then select Pressure Cook again on high for 5minutes
When the cooking program has finished for the second time, allow the pressure to release naturally for 15 minutes, then quick release any remaining pressure.
Ladle the beans into a serving dish. Sprinkle the parsley and feta cheese, then drizzle with the remaining 2 tablespoons olive oil, and serve.