I've done several rounds of Oxypowder , as well as used other magnesium based cleansers and I don't think taking it with food in the digestive tract works too well. I think it basically just liquifies any food you've eaten in the past 12 hours. The only way it's going to dissolve old fecal matter is if that's all there is in there. That's why I think it works better when taken first thing in the morning when you haven't eaten for ten or twelve hours and you've already had your morning bm. Only then can it go to work on anything stuck to your colon walls.
I also think their recommended amounts are way too high, unless you're like John Wayne (over 6', 250lbs and with a horribly packed colon.)
For periodic maintenance I just use 1 or 2 capsules first thing in the morning for a week or so. But, I get the exact same results by just taking a half to a full teaspoon of Epsom Salts for the same time period.
On the other hand, I seem to be the only person who isn't totally convinced, so what do I know. The only reason I still use it is because Andreas recommends it. Personally, I get the exact same results with Epsom Salts and it's a whole lot cheaper.