With the hairy look it almost looks like a clump of black beard algae… no idea how that would happen though.
Could have been some type of parasitic worm or crustacean - what’s important is that it’s gone now.
That parasite looked like the creature from the original alien movieGlad you were able to remove it with the tweezers as it had burrowed into the flesh of your Oscar. I agree with adding some salt to help with the healing process.