Okay. Here's what I would do. As carefully and gently as possible, catch the Oscar and wrap him in a cloth. While holding him flat on a table, open his mouth and start chopping up the pleco with scissors until you can remove most (preferable all) of it. Work very quickly so the Oscar doesn't suffocate and look into his mouth very carefully when you are chopping the pleco. He gills are at risk here. Nick them and he could bleed out and die.
*edit. Oh, he ate it?! Very nice! Yeah, he should have chewed it up pretty well on its way down.