Breathing heavily, gasping, rapid gill movement for prolonged periods of time maybe a sign that something is not quite right. A fish should not look as though it is fighting for oxygen. Obviously it will have to open its mouth but it shouldn't be a constant opening and closing motion. If you notice that you Oscar has started to struggle with its breathing then there are a few things to do at first. Firstly is your water in good condition? Check for ammonia, nitrite, nitrate and pH. If your water checks out okay make sure that your tank is being oxygenated probably. Check whether water is coming back in from your filtration, is it creating surface agitation? If not, it could just be a case of lack of oxygen in the water. If indeed this is the problem then all your fish will probably be behaving in the same way. Adjust the pipe and if all the fish stop breathing heavily you know what the problem is.
The warmer the water is, the less oxygen it will hold. This is why in the summertime when it's very hot you often see fish in ponds hanging just under the surface, often gulping for air. Check the temperature of your water, make sure that your heaters are not malfunctioning and heating the water too much. You're more likely to find the water increasing greatly in temperature during the warmer months of the year so it might be worth investing in an air pump so you are unable to increase oxygen levels in your water.
Unfortunately it may be a little more serious than just a lack of oxygen, or even toxins in the water. Breathing heavily is often a sign of stress and if a fish is ill this is exactly what it will do. Fish can suffer from all different types of illnesses, from internal parasitic and bacterial infections, to organ disorders. Heavy breathing in fish isn't always serious, but it's something you shouldn't ignore.