Hole in the head or head and lateral line errosion is caused by irritation to the pores of the fish. I have seen it commonly in oscars and geophagus as well as other fish.It most commonly comes about from insuffiscient water changes.Oscars are big eaters and need extra water changes.High nitrates are seen commonly with hole-in-the-head,alot of times over 100 ppm,if you were able to do enough water changes to keep it below 40ppm,you would probably not have it develope.It is difficult to treat after it developes,the extra water changes help the most, and generaly stops the progress.Nitrofurazone is a safe treatment that will help on infections that sometimes develope in the holes,this medication will discolor water though.I have also seen chemicals like copper irritate pores and cause errosion.I used to own a tropical fish store for 25 years and have seen alot of hole in the head.I have also seen some impressive ,healthy oscars where the owner did at least 50% water changes every 2 weeks.