Is it a good idea to put some small hardy fish like plattys in the tank to get the biological system working before putting your messy oscar in the tank ?

Oh yes. Oscars don't do well in poor quality water

i used a lot of guppy when cycling the tank. it is much easier
and cheaper to handle. once the tank is fully cycled, healthy
guppies are excellent treat for the new comer oscar since you
are sure that the guppies had been there for 4 to 6 weeks.