Oscar Fish Is Attacking Its Reflection
Have you ever seen your Oscar Fish attacking the side of the aquarium, mouth open, flaring gills, shaking its whole body? Worry not, it's probably just a case of the Oscar seeing its own reflection in the glass. Oscar Fish are not the sharpest tools in the box, they are not going to recognize their own reflection, all they are seeing is another Oscar. They will probably think that the reflection is a rival to their domain. It can also happen if they expose the bottom of the tank by shifting the substrate. I have observed my Oscars looking at their own reflection in the bottom of my aquarium and it is quite funny. I shouldn't worry, no harm will come to your fish and the chances are they will get used to it and you may only see it happen a few times and then it will stop altogether.
If you don't like seeing your fish stressing out then you could try putting a dark backing on the side and back of the tank, this may stop the fish seeing its reflection. Also, just rearrange the substrate if you see them clearing it away from the bottom of the aquarium.
But like I have already said, there really isn't anything you should worry about if you do observe your fish trying to fight its own reflection in the aquarium glass.