Well according to your source, NetMax, at least as much as 50% of their
diet would be plant matter:

http://www.fishbase.org/TrophicEco/F...cies=ocellatus

Since the rest is comprised mostly of shellfish and insects, it is
fairly safe to assume that the natural diet of the Oscar is not mostly
comprised of other fish. I concede that they are not necessarily
herbivorous, though what they may eat in captivity and what they
generally eat in their natural habitat is most likely completely
different. When the complete studies are done on wild-living oscars,
we'll all know for sure.

As to my source, I must confess that I completely forget where I first
heard it, but at the time, it was good enough to convince me of what I
already strongly suspected.

And let's not forget, that virtually any animal (including humans) can
be trained to eat a perfectly unnatural diet. It may end our lives
prematurely in the long run, but for what it's worth in the short term,
it appears to be perfectly adequate and suitable for meeting all health
requirements. I think that the lack of albino specimens in the wild is
indicative that something we are doing isn't quite right.