You can also shuttle food directly into the pleco's mouth with a plastic tube. Feed the Oscar first, then put the tube into the tank. If he investigates the tube, immediately (gently) tap his nose with it. He must know that the tube is not pleasant. Then get the tube next to the pleco's mouth and drop an algae wafer or pellet down into it. It will take a few days, but soon the pleco will dash toward the tube and wait anxiously for food, and Oscar will usually only need light reminders that the tube is unpleasant.
This is easier to do if the pleco is in a cave, but it's not strictly necessary. Just keep the tube in the tank while the pleco munches to keep Oscar away.