Looks like it is healing already. It is not too deep either.
Here is a picture to calm your woes:
You can see in the photo his lip is still messed up from when he split it in half a month or so before that. Now you cant tell
That is one great thing about Oscars, they heal amazingly fast with clean water.
If you want, you can add some aquarium salt (but not much because the pleco is salt sensitive) ... and/or grab some MelaFix/PimaFix ... get the PondCare not API sub-brand, since it is more concentrated, and you most likely WILL need it again. They will help heal and prevent secondary infections, though, it looks like it is coming along well already.
I can guarantee he jolted around at some point, either bumping a rough piece of decor, or hitting his head on equipment such at the output area of the filters, or even the intake tubes. If you have a rock or driftwood, that could the the culprit because it is a large area, but it all depends on the angle he hit at.
Within two weeks, you probably won't even be able to tell it happened, if not, by three weeks I am almost positive
The best you can do is evaluate your decor, but, even in an empty tank, they can still hurt themselves. I know, mine has done this many times just on the filter. The Penguin as some somewhat sharp corners, but anything sticking out and a fast moving Oscar, even the heater, is a potential loss of scales.
I hope I have relieved some of the stress
Your O will be just fine. Take care and try not to worry too much more.
