Rapashy Soilent Green is consumed with gusto by Ottos.
My thinking about feeding this is that they need nutrients that vegetables cannot supply and while a small number may be able to get this from the tank if you have a large number of Ottis they will rapidly exhaust the supply. Equally the jury seems to be out on their diet and some subspecies seem to be omniverous.
Recently, on another thread here on UKAPS, I mentioned that Otocinclus appear to consume periphyton, a cocktail of microflora and microfauna. For more information, take a look at:
My understanding is that Otocinclus will also eat brown 'algae' (diatoms). It's a fascinating world when you get down to the microscopic level.