Who do you like more? The pleco or the Oscar? There are many potential scenarios here.
If the Oscar is breathing alright he might survive the ordeal. You could likely do nothing and the pleco will die and once he softens the Oscar should be able to dislodge him. Alternately, the dead pleco could infect the wounds he made in the Oscar's mouth and they both die.
Or, you could take a pair of scissors and cut the pleco in half lengthwise, or cut off his fins, to get him out of the Oscar's mouth.
Or, if the pleco is still alive and you would rather rescue him and kill the oscar, you can kill the oscar and cut his mouth open to rescue the pleco.
Or....the options are many.