Ground corn is the first ingredient in Ol'Roy. This is the first few ingredients listed for it:

Ground yellow corn, meat and bone meal, corn gluten meal, soybean meal, animal fat... and so on. Lots of filler.

Here are the first few ingredients for one of Diamond's basic foods (Diamond Beef Meal and Rice):

Beef meal, brewers rice, wheat flour, chicken fat (preserved with mixed tocopherols), chicken meal, beet pulp, egg product, etc.

And their Lamb Meal and Rice:

Lamb meal, brewers rice, brown rice, rice flour, fish meal, chicken fat (preserved with mixed tocopherols), egg product, beet pulp.. etc.

Not top quality but definitely seems better than Ol'Roy to me. I know Diamond has a lower price tag than a lot of foods of its grade.