I agree with fishghost...
You have an Oscar... he his happy and comfortable in HIS aquarium... then all of a sudden... some little guy shows up expecting equal rights to the original Oscars tank. Heck no!! He thinks...
Oscars do great together when raised together from Juvenals. Really I should say babies (3 inches or under) after 3" you can add another and there are some steps you can take to make it go smoother.
If possible add them to a new tank at the same time (which in this case I'm sure isn't much of an option). The next best thing to do is to remove both fish, rearrange the decor... Change or add décor if possible…then add the 'new' Oscar. As soon as you release the 'new' Oscar immediately start floating the 'old' Oscar. You don't really want the 'new' one to make himself at home, but you do want the 'old' Oscar to realize the 'new' O was there first. The more you change the decor the less the 'old' Oscar will see it as HIS tank.
Also, as often as we hear 'you cannot have Oscars and plants', when Oscars (this will most likely apply to all plant abusing fish, but O’s for sure) become aggressive you can sometimes give them something else to exert their aggression on. And for some reason.... the love to do so with live plants. If possible/practical add a few live plants and accept your Os will tear them to shreds and kill them. Also adding live food will distract them. Basically fish don't have much to do... so if you give them something to side track them from fighting... they often will.
There are no guarantees that any of these (or other) ideas will work, but hopefully this will reduce the aggression at or between fish long enough for them to get used to each other.
Good luck!
-Toby
