welcome to OFL
Don't worry, this happens all the time. No small fish are safe with Oscars present.
www.oscarfishlover.com/index.php/compone...3/id,49/page,inline/
Anyway, that's quite a long time for it to have it stuck in its mouth, it probably will have because it will have to chew it up first. I put two bristlenose catfish in my tank and one of my Oscars try to eat one of them but couldn't because it was too big, so he spat the carcass out. I never found the other one so I'm assuming they did manage to eat it.
I think you're going to need to take this dead fish out by your self if this carries on for too much longer. If you don't, the Oscar is going to very distressed which can lead to death is stressed out too much. You'll probably need something like a pair of needle nose pliers. They're quite narrow so you can put them down the fishes mouth and grab any obstructions.
I think you're going to have take the bull by the horns here.if you feel you can't do it, or you're not getting anywhere, you might have to call the vet to do it, or just pop the fish in the bucket and take it down. Alternatively, if you know somebody who is used to picking up fish, they may be able to help