Turn on a multi-functional pressure cooker (such as Instant Pot) and select the Sauté function. Add oil, onion, garlic, and oregano; cook for 5 minutes. Stir in beans and water. Close and lock the lid. Choose high pressure and set the timer for 35 minutes. Allow 10 to 15 minutes for pressure to build.
Release pressure using the natural-release method according to the manufacturer's instructions, 10 to 40 minutes. Unlock and remove the lid. Check a bean; if not yet tender, close and lock the lid and cook for 5 more minutes at high pressure.
Drain beans, reserving 2 cups of liquid. Return beans to the pot. Add salt and 1/2 of the reserved liquid; purée using a stick blender, adding more cooking liquid as needed. Use the Keep Warm or Slow Cooker function to keep warm until ready to serve.