Hello; As indicated by others the notion of a fish that can eat the feces of another fish and clean up a problem is not known to me. Catfish will need some sort of regular food and, as stated, will add to the detritus in a tank.
The nearest I have ever come to the kind of solution you describe was with a plecostomas and some ramshorn snails. The pleco ate a lot of plant matter like other vegetarian animals. Vegetarian animals generally have an ineffecient digestive system and their feces will tend to have a lot of partially digested material with a lot of food value still left in it. For example, cow manure can be dried out and used as fuel for cook fires in places where people do not have trees any longer.
I had a large pleco that produced quite long very unsightly strings of feces. I introduced some ramnshorn snails to the tank. The snails consumed the remaining part of the pleco feces that had food value. I then had large quantities of snail feces, but it was not so unsightly and was much easier to vacuum/siphon out of the tank. As a bonus the hornwort that was in the tank thrived and the snails were numerious enough to become a source of live food for fish in the other tanks.
Oscars are more likely to be fed flesh type proteins and seem more like carnivores to me. Like cats, dogs and such their feces will have very little remaining food value. I doubt that you will find another fish that will take care of the problem for you. Oscars are also felt by myself and perhaps others to be messy feeders. You may get some benefit from snails in that they will scour the small areas of a tank where tiny bits of food can be found and consume it, which should be better than the decay process that will occur otherwise.
As indicated by another, if it were only that simple.