Id probably go with some of the medium sized pims like Sorubim kima, Pimelodus ornatus or Hemisorubim platyrynchos some earth eater type Cichlids (satanoperca or Geophagus species) and a big school of red hook silver dollars if i were limited to a 180g, if i could have a wider tank then i would add some rays into that as well.
I wouldnt recomend oscars with black arowana's, oscars can be a bit bullish and black arowana's are a bit too fragile to be bashed about by a large Cichlid.