Can you please post a photograph? Hole in the head disease doesn't normally appear from nowhere, it is something that starts off quite small and gradually gets worse if not treated. Try not to do too much of a large water change, check your water first, if everything seems okay, you needn't go too mad. Carryout water changes according to your nitrate levels, try to keep them at 20 or below. This will certainly help when it comes to the healing process.
Hold in the head disease is not something that Oscars catch. It is actually caused by a parasite that often lives within the fish. Normally, the fish lives a perfectly healthy life even though these parasites live within. However, if the fish is exposed to poor water conditions, or is fed an unsuitable diet, these parasites can cause a problem and this is when the symptoms of hole in the head disease starts appearing. This is my understanding of the disease, it is actually a disease that is not quite understood fully yet. The easiest way of preventing it is to carry out regular water changes and feed the fish a healthy diet.
