If they are a compatible matted pair, and the tank is under 6ft, you may never be able to reintroduce the oscar, or....at least not not until spawning is over and fry out of the protecting stage.
Until fry are able to fend for themselves, the pair will attack any other cichlid (fish) added to the tank.
And the reason I say maybe never, is that mated pairs often continually spawn one spawn, after another, so those type pairs think of any other fish in the tank as threats to their fry (and their pair bond).
Whenever I have had compatible pairs of any reasonably sized pairs of cichlids, those pairs usually get a 6 ft tank just to themselves.
If it ends up being a loose, or temporary, or same sex pairing, then the oscar might be allowed back in, after eggs are eaten, or simply fungus over.
