Thanks for the info and the tank shot.
It is still difficult to see clearly the area on the Oscar. I am going to attach some pictures, first of fungus, and then columnaris so you can compare these to your fish's trouble spot.

This is fungus

This is columnaris
Thanks for providing the information I requested.
It sounds like you are working hard to try to provide a good schedule of water change and feeding for your fish. Your food choices sound good, too.
One thing you do need to do is test for nitrites and for nitrates. 0 ammonia is good, but nitrites need to be 0 and nitrates at 20ppm or less.
Liquid test kit for those parameters is needed, as well.
Your tank size at 160 liters is a little more than 42 gallons, which is a bit of a problem. I'm not an Oscar expert, but I have read many, many posts by Oscar experts, and the consensus is that Oscars need really big tanks.
It seems that technically, based on bioload alone, 50 gallons is the minimum per Oscar, but that does not take into consideration tank dimensions. A standard 55 gallon long tank is not even adequate, because the fish will grow and not be able to turn his body around due to the narrow depth from front to back.
The consensus seems to be that an Oscar really needs a very large tank.
This link has a discussion about tank size by members who are Oscar keepers.
http://www.aquariacentral.com/forums/showthread.php?t=163778&highlight=Oscar&page=8
You have three Oscars and a pleco, if a common pleco they get quite large, too, and need a huge tank as well.
Water quality may be an issue with the problem your fish is having. I would isolate him in a quarantine tank, if you can, to protect the others in the tank.
Whatever is on your fish's head certainly may be helped by having pristine water conditions first and foremost, and I would start there until someone with better skills at diagnosing comes along that can suggest other treatment options. I've dealt with bacterial infections, and that may be what you have, but it could be HITH.
Until you can reduce the number of fish in that tank, and/or get them a huge tank, I would take extreme measures to help insure their health.
I would reduce the temperature of your tank in case this is columnaris. Columnaris likes warm temperatures and cooler may help to impede it, somewhat, if this is bacterial. I've read that the comfort range for Oscars is 74 to 81 degrees F, so I'd go to the lowest comfortable temperature for him. (I don't know the celcius conversion off the top of my head but you can look it up),
I would increase your water change to at least 50% every two to three days, and I would over filter the tank. I forgot to ask what size and what kind of filtration you have, but I would filter that tank as if it were a 300 gallon, to help reduce the effects of the bioload you have.
The appropriateness of the number and size of fish in a tank is more than just a visual matter. In other words, your fish are still fairly small and to the fishkeeper the tank may not look overcrowded and the fish may not look to big for the tank, yet, but there's more to it than that.
Fish produce some kind of biochemical messenger based on tank size and other occupants that give them a sort of feed back, as it were, about the tank size and crowding.
They begin to be affected by this feedback and this causes them to become stunted in their growth long before it is apparent to the fishkeeper, and this can contribute to other health problems, as well.
I would try to rehome some of the fish if you are not able to get much larger tanks for them.
I'm not meaning to be critical, I'm just trying to help

and I'm merely repeating what I've learned from the fishkeeping experts here on AC.
Quarantining the sick fish is very important, not just to prevent infection of the others, but, also, a fish that is sick or has a lesion or trauma to the body is likely to be a victim of aggression by the others in the tank.
I hope these suggestions will be helpful, until someone that is more knowledgeable than me comes along and can provide more help.
If you could try to get a good sharp closeup of the fish's head and lesion, it would be helpful.
As someone suggested, try getting a picture while feeding him. You could hold a tasty bit of food over the tank near him while someone else takes his picture. Maybe we can get a better look at the problem area.
