Originally Posted by OrangeSoda
It's a territorial dispute. Oscar sees the "new" guy as an invader in his home. Remove both of them to separate containers. Then rearrange the tank. Afterwards put the new guy in first, then Oscar. They will carve out new territories as they are "no longer in the same place".
If that doesn't work then it may just be that Oscar is just really aggressive (which I don't think is the case as you didn't seem to have an aggression issue with the original other gourami). If he is just aggressive, then add more tall structures to the tank that will break line of sight during a chase.
What size is the tank musician? That is actually the biggest question as in to small of a tank and there will be no possible way to "fit two territories" in the space.
10 gallon. Don't have $$ or room for a bigger one right now =/