Your other fish will poo too you know
All plecs poo a lot - it's a common misconception that they keep your tank clean - as although they will clean up some algae, they will make twice as much mess doing it

Contrary to what's said in an earlier reply - not all plecs eat bogwood, but all plecs poo a lot - colour could be anything they've eaten.
If you do regular gravel vacs anyway as part of your normal clean up, a plec shouldn't cause too many problems

If a lot of waste bothers you, get a smaller one. But to be fair - all fish poo, so it's likely you'll still have some waste around the tank whether or not the plec is in there
I'd say your oscar will be even worse, but then he might end up needing a new home too...this is why people should research their fish
before they buy them! If you dont like poo - you made a really daft choice getting an oscar and a plec...
Edited to add - how big is your temporary tank, and how big is your permanent one going to be?