Not all Otocinclus adjust to processed foods. Sewellia seem to. In nature, they eat mainly biofilm and algae. Check them on Seriously Fish for greater detail.
Seriously fish is probably the best source of information about fish in the hobby, and a good placer to visit to research before you get any fish you are not familiar with. They already gave you a link in one of your other threads, you should spend some time browsing the site. Not sure why you say that it doesn't have anything... just as an example, here is information about otos -- you can see diet information about halfway down (It's about otocinclus cocamas, but the same info about diet applies to any oto).