It isn't ICK. Never seen that on an oscar, but it could just be a color change, which sometimes happens as fish age. I would do a 50% water change and treat with Prime, and see what it looks like in a few days. If it looks worse, you'll have to find out what it is and treat accordingly. On the other hand, if it hasn't worsened, and your oscar is eating, and behaving normally, it may be nothing to worry about.