When I feed my oscar feeders sometimes he has part of its body and head sticking out in its mouth for a while. If he's pretty much all in don't worry about it. The O will swallow it fully in time. They do that sometimes.
1. Does the Oscar look like it's struggling?
2. Is the shark too big to completely ingest fully?
"Also why did it he do it?"
Why NOT?
Oscar's are predatory fish.
If he
can fit it in his mouth there's a good chance he will try at some point. Even fish who are supposedly able to 'get along' with O's...if they are significantly smaller...well, you can't ever really predict what he'll decide to do.
Just REMEMBER:
One fish goes one way, the other fish goes the other way, and this guy's sayin', "Whadda ya want from me?'