Hey Tweaks!
First, please know that I am not trying to confirm or deny what exact illness/disease this fish really has...but only passing along what I know on Hole-in-the-head disease. Also, I admit that I have not encountered this problem myself, but I have read a fair amount about it and how others treated for it.
From what I understand, this problem is not really something that you can solve or treat for by plopping some 'fizzy tabs' into your tank and walking away. In fact, this illness is most often 'brought on' as a secondary effect to poor water quality and poor diet. After confirming that by googling up some information on the subject, you will most likely want to find a way to supplement this fish's diet with Vitamin C and a few other items. That resource I mentioned suggests feeding more veggie-based foods as well as to do multiple small water changes in succession...which probably means something like doing a 10% water change once a day or every other day. The reason that you really don't want to do one big water change is probably based upon reducing the risk of osmotic shock to an already stressed fish.
HOWEVER, that website also states that hole-in-the-head disease is often misdiagnosed because it is similar to another issue; Hexamita. The reason I bring this up is because it was clearly stated that a few fish are more apt to have this "hole in the head-like" versus others, and of course, Oscars are on that list. Keeping in mind that I often times try not to use meds or other chemicals in my tanks, I have to say that the good thing here is that you might be able to treat for both at the same time since one treatment would be antibiotics while the other is just improving water quality and diet...hence, you wouldn't be putting two types of meds in the tank at the same time. Along the same lines, it may be somewhat good to know that this could not be the only problem you are facing since wounds like this are more easily infected by bacteria and/or fungus. Again, I don't like to suggest that anyone should always rely on meds, but in this case, the antibiotic from the Hexamita treatment (if you even go that route) might also protect from an infection.
I hope this helps at least a little bit. I would find it hard to believe that no other member could help you out more than I can here so keep an eye on your questions here. Also, if you would like, maybe moving this question into the
Freshwater Fish Diseases area might get you some better replies from those members who know about this topic. If you do, just let me know.