Oscars are cichlids. They get defensive as mentioned above. If any newcomers in comes into their territory they will try to act tough and push the other fish around. With other cichlids, the oscar usually gets pushed around, but anyways...
He may also see the shark as food as Oscars will put anything in their big mouths that can fit, sorta like a kid.... so you have to be careful with the sizes of the other fish and inverts and the such.....
Another thing about balas is that the bala may being aggressive towards the oscar causing the o to get defensive. You may say, everything I read says balas are peaceful, and this is true when they are in shoals, but one or two lone balas will get aggressive towards anything that moves as they feel very vulneralb ena uncomfortable. I saw it first hand when two of my balas became sick when i had them and had to be quarentined. The two left in the tank began nipping at other fish and charging them to stay away. It was quite shocking to me but on more reasearch, found that this is common for lone or small groups (2 or 3) of balas. Lesson: Balas need shoals of 6+ hence the need for atleast a 125G tank.
So, you should have your O in a 90G atleast anyways as this is what I call minimum, even though some say a 75G is ok for a single adult, but if you have it in something like a 100G or 125G then buy a few more balas (of good size as to not become snacks for the O) to help calm the lone bala and make him feel more secure. If you have 90G or less, then the bala isnt going to work and needs to go back to the lfs if you cant get a bigger tank.
After awhile the O should settle down with his aggression towards the bala once it realizes its of no threat and not going anywhere but still watch carefully to make sure that the bala doesnt get beat up. If the O continues to beat on him, tanother reason why the bala needs to go and your fish is too defensive to add new fish, sorry.... my JD is the same way, i wouldnt dare add a new fish to the tank