Measure out a 40-gram serving of oats (1/3 cup).
Add a little over 1 1/2 cups liquid.
Microwave or cook on the stove, but stop before all the water is absorbed. (I microwave for 4 minutes, then leave in the closed microwave for 5 more minutes.)
Put in the fridge, uncovered. When you return a few hours later, the watery mess above will have magically turned into 2 cups-worth of pillowy, voluminous oats!
Now, I like my oatmeal like hot pudding!
I added 3T cocoa and 1T cinnamon, also 1T vanilla essence. Then I re-heat 2 minutes in microwave until it gets hot, no longer, and the volume will stay.