For what it is worth, this is a very common occurrence, so you are definitely not alone. I couldn't tell you exactly why your fish have decided to start fighting. There could be various reasons, it may well be a dominance issue, the larger oscar wants to become top dog in the tank, this does happen a lot, you often find that if you have the same species of fish, one of them is more dominant over the others and they will often back down at feeding time and maybe quite reclusive sometimes. Quite often, some fish will even pick on different species of fish in the tank as well. It is not natural to have fish in such a small environment so it's not surprising aggression breaks out quite a lot.
The best advice I can give you is to keep a close eye on them, then use your discretion. If you believe that the OSCAR is going to come to serious harm then you may have to re-home one of them. You've already rearranged your tank, this is something that works sometimes, but more often makes absolutely no difference whatsoever.
You may find that they calm down a little after a period of time.
