I was looking on google about oscar fish diseases and i saw this one in the image and couldn't tell what it is. I look on the link for the image but it was a old thread from another forum that was unanswered.
I'm testing you!
It is not ich. Ich looks like salt crystals. I would suggest it is fin rot of a fungal nature. I would guess that one of the spines in the fin being damaged created the opportunity for the fungas to take hold. Often there is a bacterirail infection whihc comes befor the fungus but I saw no signs of that in the picture.
As always this is just my opinion (except for it not being ich).