Put simply magnesium oxides work by drawing water into the colon, the have a kind of osmetic effect and flush water through.
Oxypowder works completely differently, the avaliable oxygen is used to actively oxidise and remove matter, it destroys anarobic bacteria (the bad kind) and creates a more stable environment for your own good intestinal bacteria. Its a very efficient intestinal cleanser indeed, but it shouldnt be the only way you cleanse.