Press the sauté button on the pressure cooker, and adjust the temperature to high. When the sauté function has preheated to hot, coat the inside of the pressure cooker inset with non-stick cooking spray.
Add the onion to the hot pan, and cook for 4 to 5 minutes, or until the onion is translucent, stirring frequently.
Add the spinach, artichoke hearts, oregano, and water.
Seal the lid of the pressure cooker, close the valve, cancel the sauté function, and reset the pressure cooker to Manual/Pressure Cook for 6 minutes.
Once the cooking has completed, quick release the pressure. After the valve drops, carefully remove the pressure cooker lid.
Stir in the cream cheese pieces, cayenne pepper, salt, and garlic powder. Stir until the cream cheese completely melts, breaking down any large artichoke pieces while stirring.
Add the shredded cheddar cheese, and stir until melted.
If the soup is too thick, feel free to add a little hot water or milk to thin it.