Have kept oscars for many years now, "hole in the head" is very commom and very easy to treat.
My oscars seem to get it after feeding them live food such as goldfish etc.
You can get a tablet type of medicine (sorry, can't remember the name ) specifically for hole in the head, & if you get a dead goldfish and insert the tablet into that, Mr oscar will eat the whole lot! job done!!

Don't worry too much , hole in the head is very common.

Hope this helps,

Nick.