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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.
Very difficult to diagnose this. Knowing something about large cichlids, my first instinct would be to increase the water changing. It's amazing how improving the water quality can cause a rebound in health.