Unfortunately oscars will often take a dislike to new tank mates, loads of people have the same problem. It seems that some oscars and more placid than others and will often be accepting of other tank mates. However, mixing aggressive or semi-aggressive fish is probably more of a risk than adding non-aggressive fish like silver dollars, earth eaters etc. Whereas all my fish seem to live together in harmony, the only other tank mates that my oscars will often have a little bit of banter with are the other semi aggressive cichlids I have got, they don't bother with the silver dollars, leporinus, or earth eaters. You'll just have to give them a little bit of time, you may find that everything settles down, then again, the aggression may just carry on.