Rick & Lucy,
I think I can see what you are talking about, and it is probably from fighting. When you have two oscars together, they can pick on each other, and so I would assume this is a probably a wound from that. As long as there are no other issues, a few water changes and it should heal its self. However that doesn't mean that it won't happen again, or there won't be some other type of injuries from these fish fighting. Your substrate/gravel is huge, and if they are "playing" around rubbing on this gravel that could also cause injuries.
Keep an eye on it, if it doesn't clear up, you will need to isolate and treat the fish.