Dry cat food probably won't harm your Os. But, it also won't do them much good, either. The crude protein levels for most brands of cat foods are in the low to mid 30 percent range. Which is about 10% lower than alot of economical fish foods. Fat levels can cause you some maintenance issues since cat food contains the mid 20% range in fats. This is about 10% higher than the fat levels in fish foods. And the ash/filler levels run an average 7%.
I have personally seen an oscar raised on nothing but dry cat food and it aint pretty. I used to give it to mine on occasion just to give em something new but that was only once a month. Maybe even less. If you're looking for something different for your oscars try some unpeeled frozen shrimp or maybe some mealworms, waxworms or crickets.