I agree with
tlindsey
, the 30" of the tank each fish would be left with, will not be enough room. The Oscar can grow to half that length, would not be enough room to swim around.
I think you will have lots of trouble introducing another fish to live with your red devil. The devil has already established the tank as it's territory, and will not allow another large cichlid, which would be seen in the wild as a competitor, to share that space.
I would just let the red devil live alone in that tank personally. If you were really determined to get some other fish in there, you could try completely re-arranging the tank (putting the RD in a bucket for this could help too) and putting in a few male convict cichlids. They are smaller than the RD and look quite different, so hopefully he will not see them as a threat. That said, there is still a good chance he will attack anything you put in with him.
Best of luck