Melt coconut oil in a multi-functional pressure cooker (such as Instant Pot®) set to the Saute function. Add oats and toast until they are starting to brown, 2 to 3 minutes. Turn off Saute function. Add water, almond milk, zucchini, cinnamon, nutmeg, and salt. Stir lightly.
Close and lock the lid. Select high pressure according to manufacturer's instructions; set timer for 4 minutes. Allow 10 minutes for pressure to build.
Release pressure using the natural-release method according to manufacturer's instructions, about 10 minutes. Release remaining pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove the lid. Stir in maple syrup and vanilla extract.
Divide oatmeal evenly between 4 bowls. Top each bowl with 1 tablespoon walnuts.