That really is a clear picture!
It should make it alot easier to diagnose the problem. It looks to me like a few scales are missing. Perhaps your oscar scraped himself and the wound is healing.
The only other thing I can think of is early Lymphocystis, or Cauliflower Disease. It's a viral infection that produces fluid-filled, caulifower-shaped cysts (analogous to a wart). From what I've read, really clean water is the best treatment - reduces stress and allows the fish' immune system to better fight the virus.
Let's see what the others have to say.