I didn't use Oxypowder but I did use Colosan which is basicly the same thing. They both dissolve old fecal matter and create oxygen in the intestines. I usually felt energy a few hours after taking a dose.
I understand it to work like this: The old matter dissolves away and oxygen is absorbed through the walls of the intestines. The oxygen is what gives that boost in energy. As the intestines become cleaner, the person naturally feels better and has more energy.
If the person has a lot of gunk built up there probably wouldnt be much oxygen getting through to the walls to be absorbed by the body. It seems that after some use, say several days, they might start to feel that energy.
I guess there is the possibility that someone is already clean and taking oxygen via ozone or h2o2 so therefore feels no difference.
Cleaning out our intestines is a big step towards total health.