i have a tiger oscar, a jd, a firemouth a plec and strangely a krib and an angel fish all sharing a tank hapily (the last 2 were only added as a last resort due to space/friendsa poor selection) supprisingly the jd and oscar both eat from the same place but the oscar seems fine about the whole thing, aslong as theres enough food for her she see's no threat by the jd. the jd was added when the oscar was well over a year old and well ove rhalf size. a 2" juvinile jd was added, a few weks later the firemouth was added. the main supprise was the addition of the orphaned angelfish, wich i thought would honestly become food, but has taken spot as number 2 in the tank to the oscar. ive found wiht oscars that its all down to the temperament of the fish, and sometimes the only way to find out is to try.
finding tankmates that are compatible with eachother aswell as the oscar will be the hard part. but again, your tank is probably a little small for any experimental or partially aggressive mates. severum's ive allways found friendly yet big enough to not be seen as food by the oscar. some oscars love friends, some are loners. this is the joy of the river dog. they each have a different personality.
alternativly a small pack of tiger barbs work well wiht a moody oscar, they both leave each other well alone providing there are enough tiger barbs (6 or 8) as they fear the oscar yet are faster so wont be eaten