Place oats in a multi-functional pressure cooker (such as Instant Pot(R)). Sprinkle with pumpkin pie spice and salt. Add water, milk, pumpkin puree, and cranberries. Stir lightly.
Close and lock the lid. Set to Manual; select high pressure according to manufacturer's instructions. Set timer for 3 minutes. Allow 10 to 15 minutes for pressure to build. Keep warm for 5 minutes more after cooking time is complete.
Release pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove lid. Stir in honey; divide oatmeal into 4 bowls. Top each serving with 1 tablespoon walnuts and 1 1/2 teaspoons brown sugar.